md-cluster: fix NULL pointer dereference in process_metadata_update

md-cluster: fix NULL pointer dereference in process_metadata_update

The function process_metadata_update() blindly dereferences the 'thread'
pointer (acquired via rcu_dereference_protected) within the wait_event()
macro.

While the code comment states "daemon thread must exist", there is a valid
race condition window during the MD array startup sequence (md_run):

1. bitmap_load() is called, which invokes md_cluster_ops->join().
2. join() starts the "cluster_recv" thread (recv_daemon).
3. At this point, recv_daemon is active and processing messages.
4. However, mddev->thread (the main MD thread) is not initialized until
   later in md_run().

If a METADATA_UPDATED message is received from a remote node during this
specific window, process_metadata_update() will be called while
mddev->thread is still NULL, leading to a kernel panic.

To fix this, we must validate the 'thread' pointer. If it is NULL, we
release the held lock (no_new_dev_lockres) and return early, safely
ignoring the update request as the array is not yet fully ready to
process it.

Link: https://lore.kernel.org/linux-raid/20260117145903.28921-1-jiashengjiangcool@gmail.com
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>