rbd-mirror: release lock before calling m_async_op_tracker.finish_op()

rbd-mirror: release lock before calling m_async_op_tracker.finish_op()

m_async_op_tracker.finish_op() in InstanceReplayer::start_image_replayers
may invoke a completion that re-enters code paths that attempt to acquire
the same mutex (m_lock), violating the non-recursive lock constraint.
This can be fixed by releasing the lock before calling
m_async_op_tracker.finish_op().

Fixes: https://tracker.ceph.com/issues/70951
Signed-off-by: VinayBhaskar-V <vvarada@redhat.com>