TrackedOp: Removed redundant lock in OpTracker::_mark_event()

ops_in_flight_lock seems redundant in OpTracker::_mark_event()
and this lock is highly contended for. Removing the same
is giving a significant performance boost.

Signed-off-by: Pavan Rallabhandi <pavan.rallabhandi@sandisk.com>
TrackedOp:_dump_op_descriptor is renamed to _dump_op_descriptor_unlocked

Caller don't need to hold lock before calling _dump_op_descriptor(),so,
to reflect this it is renamed to _dump_op_descriptor_unlocked().

Signed-off-by: Pavan Rallabhandi <pavan.rallabhandi@sandisk.com>
Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge pull request #2219 from somnathr/wip-optracker-lock

TrackedOp: Removed redundant lock in OpTracker::_mark_event()

Reviewed-by: Sage Weil <sage@redhat.com>