libblock: Add read-ahead task

Read-ahead requests were previously executed in the context of the
reading task.  This blocks the reading task until the complete read
with read-ahead transfer is finished.  A read-ahead task is introduced
to off-load the read-ahead transfer.  This allows the reading task to
work with the requested block more quickly.  The read-ahead is triggered
after two misses of ascending consecutive blocks or a read hit of a
block read by the most-recent read-ahead transfer.  The read-ahead
feature is configurable and can be disabled.

cpukit/sapi/include/confdefs.h

@@ -1241,6 +1241,10 @@ rtems_fs_init_functions_t    rtems_fs_init_helper =
     #define CONFIGURE_BDBUF_BUFFER_MAX_SIZE \
                               RTEMS_BDBUF_BUFFER_MAX_SIZE_DEFAULT
   #endif
+  #ifndef CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY
+    #define CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY \
+                              RTEMS_BDBUF_READ_AHEAD_TASK_PRIORITY_DEFAULT
+  #endif
   #ifdef CONFIGURE_INIT
     const rtems_bdbuf_config rtems_bdbuf_configuration = {
       CONFIGURE_BDBUF_MAX_READ_AHEAD_BLOCKS,
@@ -1253,11 +1257,14 @@ rtems_fs_init_functions_t    rtems_fs_init_helper =
       CONFIGURE_BDBUF_TASK_STACK_SIZE,
       CONFIGURE_BDBUF_CACHE_MEMORY_SIZE,
       CONFIGURE_BDBUF_BUFFER_MIN_SIZE,
-      CONFIGURE_BDBUF_BUFFER_MAX_SIZE
+      CONFIGURE_BDBUF_BUFFER_MAX_SIZE,
+      CONFIGURE_BDBUF_READ_AHEAD_TASK_PRIORITY
     };
   #endif
 
-  #define CONFIGURE_LIBBLOCK_TASKS (1 + CONFIGURE_SWAPOUT_WORKER_TASKS)
+  #define CONFIGURE_LIBBLOCK_TASKS \
+    (1 + CONFIGURE_SWAPOUT_WORKER_TASKS + \
+    (CONFIGURE_BDBUF_MAX_READ_AHEAD_BLOCKS != 0))
 
   #define CONFIGURE_LIBBLOCK_TASK_EXTRA_STACKS \
     (CONFIGURE_LIBBLOCK_TASKS * \