mds: fix replay locking

When replaying EImportFinish/EFragment event, the replay thread may call
MDS::queue_waiters. MDS::queue_waiters() requires its caller to hold the
mds_lock. Otherwise assert(waiter_mutex == __null || waiter_mutex->is_locked())
in Cond::Signal() will be tiggered.

Signed-off-by: Yan, Zheng <zyan@redhat.com>