os/bluestore: drop deferred_submit_lock

os/bluestore: drop deferred_submit_lock

This lock serves no purpose.  It only protects the call to the aio_submit
on the OpSequencer running batch, but we are the only caller who will do
that submission, and a new batch won't be moved into place until ours is
completed and retired (by the completion for the aio we are queueing).

More importantly, this fixes a deadlock:

- thread A submits aio, but the queue is full, and blocks
- thread B blocks taking deferred_submit_lock while holding deferred_lock
- aio completion thread blocks trying to take deferred_lock, and thus
no further aios are drained.

Fixes: http://tracker.ceph.com/issues/21171
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 7a5ef62af194373c3a8c307b05ae6d467e49c9f0)