os/bluestore: fix deferred_aio deadlock

os/bluestore: fix deferred_aio deadlock

We submit aios while holding deferred_lock, and we take deferred_lock in
the aio completion handler (of which there is only 1).  That means that
if the aio queue fills up, the submitter will block with the lock while
the finisher won't be able to take the lock, progress, and drain completed
aios, leading to a deadlock.

Fix this by submiting aios *without* deferred_lock held.  Instead, demote
to a deferred_submit_lock.  Further, in the finisher, submit aios
via a finisher (this only happens when in deferred_aggressive mode which
is rare and not performance-sensitive).

Signed-off-by: Sage Weil <sage@redhat.com>