drivers/rpmsg: fix struct alignment issue by removing zero array member

This commit fixes a memory alignment issue in the rpmsg subsystem.
The previous implementation used a flexible array member (rdev[0])
in struct rpmsg_s, but due to automatic 8-byte padding, rpmsg->rdev
could differ from the transport layer's rdev address (e.g.,
rpmsg_virtio->rvdev.rdev), causing potential memory corruption.

Changes:
- Remove rdev[0] flexible array member from struct rpmsg_s
- Add rpmsg_get_rdev_by_rpmsg() inline helper to calculate rdev
  address as (rpmsg + 1), ensuring correct pointer arithmetic
- Update all rpmsg->rdev references to use the new helper function
- Fix rpmsg_virtio_notify_wait() to use correct container_of macro

Signed-off-by: Yongrong Wang <wangyongrong@xiaomi.com>

include/nuttx/rpmsg/rpmsg.h

@@ -68,7 +68,6 @@ struct rpmsg_s
   struct rpmsg_endpoint        test;
 #endif
   atomic_int                   signals;
-  struct rpmsg_device          rdev[0];
 };
 
 /**
@@ -108,6 +107,17 @@ extern "C"
 #define EXTERN extern
 #endif
 
+static inline FAR struct rpmsg_device *
+rpmsg_get_rdev_by_rpmsg(FAR struct rpmsg_s *rpmsg)
+{
+  if (!rpmsg)
+    {
+      return NULL;
+    }
+
+  return (FAR struct rpmsg_device *)(rpmsg + 1);
+}
+
 int rpmsg_wait(FAR struct rpmsg_endpoint *ept, FAR sem_t *sem);
 int rpmsg_post(FAR struct rpmsg_endpoint *ept, FAR sem_t *sem);