rbd-mirror: take MirrorStatusUpdater lock by value in queue_update_task

queue_update_task() took the lock by rvalue reference, so its early
returns never released it. In handle_update_task() that left m_lock
held across the on_finish->complete() calls; during shutdown a
completion can re-enter try_remove_mirror_image_status() and re-lock
m_lock, deadlocking.

Take the unique_lock by value so it unlocks on every return path.

Fixes: https://tracker.ceph.com/issues/78047
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>