md: Don't ignore read-only array in md_check_recovery()

Usually if the array is not read-write, md_check_recovery() won't
register new sync_thread in the first place. And if the array is
read-write and sync_thread is registered, md_set_readonly() will
unregister sync_thread before setting the array read-only. md/raid
follow this behavior hence there is no problem.

After commit f52f5c71 ("md: fix stopping sync thread"), following
hang can be triggered by test shell/integrity-caching.sh:

1) array is read-only. dm-raid update super block:
rs_update_sbs
 ro = mddev->ro
 mddev->ro = 0
  -> set array read-write
 md_update_sb

2) register new sync thread concurrently.

3) dm-raid set array back to read-only:
rs_update_sbs
 mddev->ro = ro

4) stop the array:
raid_dtr
 md_stop
  stop_sync_thread
    set_bit(MD_RECOVERY_INTR, &mddev->recovery);
    md_wakeup_thread_directly(mddev->sync_thread);
    wait_event(..., !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))

5) sync thread done:
 md_do_sync
 set_bit(MD_RECOVERY_DONE, &mddev->recovery);
 md_wakeup_thread(mddev->thread);

6) daemon thread can't unregister sync thread:
 md_check_recovery
  if (!md_is_rdwr(mddev) &&
      !test_bit(MD_RECOVERY_NEEDED, &mddev->recovery))
   return;
  -> -> MD_RECOVERY_RUNNING can't be cleared, hence step 4 hang;

The root cause is that dm-raid manipulate 'mddev->ro' by itself,
however, dm-raid really should stop sync thread before setting the
array read-only. Unfortunately, I need to read more code before I
can refacter the handler of 'mddev->ro' in dm-raid, hence let's fix
the problem the easy way for now to prevent dm-raid regression.

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Closes: https://lore.kernel.org/all/9801e40-8ac7-e225-6a71-309dcf9dc9aa@redhat.com/


Fixes: ecbfb9f1 ("dm raid: add raid level takeover support")
Fixes: f52f5c71 ("md: fix stopping sync thread")
Cc: stable@vger.kernel.org # v6.7+
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240201092559.910982-3-yukuai1@huaweicloud.com