mds: switch submit_mutex to fair mutex for MDLog
The implementations of the Mutex (e.g. std::mutex in C++) do not
guarantee fairness, they do not guarantee that the lock will be
acquired by threads in the order that they called the lock().
In most case this works well, but in overload case the client
requests handling thread and _submit_thread could always successfully
acquire the submit_mutex in a long time, which could make the
MDLog::trim() get stuck. That means the MDS daemons will fill journal
logs into the metadata pool, but couldn't trim the expired segments
This will switch the submit_mutex to fair mutex and it could make
sure that the all the submit_mutex waiters are in FIFO order and
could get a change to be excuted in time.
From Patrick's comment in https://github.com/ceph/ceph/pull/44180#pullrequestreview-1174516711.