bdrv_subtree_drained_end() requires the caller to hold the AioContext
lock for the drained node. Not doing this for nodes outside of the main
AioContext leads to crashes when AIO_WAIT_WHILE() needs to wait and
tries to temporarily release the lock.

Fixes: 3908b7a8
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2046659


Reported-by: Qing Wang <qinwang@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20220203140534.36522-2-kwolf@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>