md-cluster: fix NULL pointer dereference in process_metadata_update

In the Linux kernel, the following vulnerability has been resolved:

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.