fs/timerfd: implement TFD_TIMER_CANCEL_ON_SET to detect clock changes

Implement Linux-compatible TFD_TIMER_CANCEL_ON_SET flag for timerfd to
allow applications to detect discontinuous changes to CLOCK_REALTIME.

Background:
According to Linux timerfd_create(2) man page, when a timerfd is created
with CLOCK_REALTIME and TFD_TIMER_CANCEL_ON_SET flag is specified, the
read() operation should fail with ECANCELED if the real-time clock
undergoes a discontinuous change. This allows applications to detect
and respond to system time changes.

Implementation:
1. Add clock notifier infrastructure (clock_notifier.h/c) to notify
   interested parties when system time changes
2. Implement TFD_TIMER_CANCEL_ON_SET flag support in timerfd
3. Register timerfd with clock notifier when flag is set
4. Cancel timer and return ECANCELED when clock change is detected
5. Notify clock changes in:
   - clock_settime()
   - clock_initialize()
   - clock_timekeeping_set_wall_time()
   - rpmsg_rtc time synchronization

Changes include:
- New files: include/nuttx/clock_notifier.h, sched/clock/clock_notifier.c
- Modified: fs/vfs/fs_timerfd.c to handle TFD_TIMER_CANCEL_ON_SET
- Modified: clock subsystem to call notifier chain on time changes
- Modified: rpmsg_rtc to notify time changes during sync

Use case example:
Applications using timerfd with absolute time can now detect when
system time is adjusted (e.g., NTP sync, manual time change) and
take appropriate action such as recalculating timeouts or updating
scheduled events.

Reference: https://man7.org/linux/man-pages/man2/timerfd_create.2.html

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

include/nuttx/clock_notifier.h

@@ -0,0 +1,83 @@
+/****************************************************************************
+ * include/nuttx/clock_notifier.h
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+#ifndef __INCLUDE_NUTTX_CLOCK_NOTIFIER_H
+#define __INCLUDE_NUTTX_CLOCK_NOTIFIER_H
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <time.h>
+
+#include <nuttx/notifier.h>
+
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Public Function
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name:  register_clock_notifier
+ *
+ * Description:
+ *   Add notifier to the clock_notifier chain
+ *
+ * Input Parameters:
+ *    nb - New entry in notifier chain
+ *
+ ****************************************************************************/
+
+void register_clock_notifier(FAR struct notifier_block *nb);
+
+/****************************************************************************
+ * Name:  unregister_clock_notifier
+ *
+ * Description:
+ *   Remove notifier from the clock_notifier chain
+ *
+ * Input Parameters:
+ *    nb - Entry to remove from notifier chain
+ *
+ ****************************************************************************/
+
+void unregister_clock_notifier(FAR struct notifier_block *nb);
+
+/****************************************************************************
+ * Name:  clock_notifier_call_chain
+ *
+ * Description:
+ *   Call functions in the clock_notifier chain.
+ *
+ * Input Parameters:
+ *    action - Value passed unmodified to notifier function
+ *    tp - Pointer of current timespec passed unmodified to notifier function
+ *
+ ****************************************************************************/
+
+void clock_notifier_call_chain(unsigned long action,
+                               FAR const struct timespec *tp);
+
+#endif /* __INCLUDE_NUTTX_CLOCK_NOTIFIER_H */

include/sys/timerfd.h

@@ -35,10 +35,10 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
-#define TFD_NONBLOCK O_NONBLOCK
-#define TFD_CLOEXEC  O_CLOEXEC
-
-#define TFD_TIMER_ABSTIME TIMER_ABSTIME
+#define TFD_NONBLOCK            O_NONBLOCK
+#define TFD_CLOEXEC             O_CLOEXEC
+#define TFD_TIMER_ABSTIME       TIMER_ABSTIME
+#define TFD_TIMER_CANCEL_ON_SET 2
 
 /****************************************************************************
  * Public Type Declarations