md: fix stopping sync thread

Currently sync thread is stopped from multiple contex:
 - idle_sync_thread
 - frozen_sync_thread
 - __md_stop_writes
 - md_set_readonly
 - do_md_stop

And there are some problems:
1) sync_work is flushed while reconfig_mutex is grabbed, this can
   deadlock because the work function will grab reconfig_mutex as well.
2) md_reap_sync_thread() can't be called directly while md_do_sync() is
   not finished yet, for example, commit 130443d6 ("md: refactor
   idle/frozen_sync_thread() to fix deadlock").
3) If MD_RECOVERY_RUNNING is not set, there is no need to stop
   sync_thread at all because sync_thread must not be registered.

Factor out a helper stop_sync_thread(), so that above contex will behave
the same. Fix 1) by flushing sync_work after reconfig_mutex is released,
before waiting for sync_thread to be done; Fix 2) bt letting daemon thread
to unregister sync_thread; Fix 3) by always checking MD_RECOVERY_RUNNING
first.

Fixes: db5e653d ("md: delay choosing sync action to md_start_sync()")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231205094215.1824240-4-yukuai1@huaweicloud.com