ocfs2/cluster: hold a reference on the heartbeat thread

Since commit 688bc88e2046 ("ocfs2/cluster: keep heartbeat local node
stable"), o2hb_thread() leaves its loop and returns when the local node
changes, for example after "echo 0 > node/<name>/local".  The thread was
started with kthread_run() and nothing holds a reference to its
task_struct, so the task is freed once it exits, while reg->hr_task still
points to it.

Reading the region's pid attribute then reads the freed task, and removing
the region calls kthread_stop() on it:

  BUG: KASAN: slab-use-after-free in o2hb_region_pid_show+0xb3/0xc0
  refcount_t: addition on 0; use-after-free.
  Oops: Oops: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:kthread_stop+0xb1/0x390

The thread could already return by itself before, when heartbeat start was
aborted or on an unclean stop, but the local node change makes it
reachable from userspace at any time.

Create the thread, take a reference on it and only then wake it, so the
reference cannot race with the thread exiting.  Drop it with
kthread_stop_put().

Link: https://lore.kernel.org/20260923010149.14391-1-kmehltretter@gmail.com
Fixes: 688bc88e2046 ("ocfs2/cluster: keep heartbeat local node stable")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260616074931.3774929-1-zzzccc427%40gmail.com
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Assisted-by: LLM
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Cen Zhang <zzzccc427@gmail.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Heming Zhao <heming.zhao@suse.com>

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