md: unlock mddev before reap sync_thread in action_store

mainline inclusion
from mainline-v6.0-rc1
commit 9dfbdafd
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6OMCC
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.3-rc3&id=9dfbdafda3b34e262e43e786077bab8e476a89d1



--------------------------------

Since the bug which commit 8b48ec23 ("md: don't unregister sync_thread
with reconfig_mutex held") fixed is related with action_store path, other
callers which reap sync_thread didn't need to be changed.

Let's pull md_unregister_thread from md_reap_sync_thread, then fix previous
bug with belows.

1. unlock mddev before md_reap_sync_thread in action_store.
2. save reshape_position before unlock, then restore it to ensure position
   not changed accidentally by others.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>