ublk: rename mm_lock as lock

Rename mm_lock field of ublk_device as lock, so that this lock can be reused
for protecting access of ub->ub_disk, which will be used for simplifying
ublk_abort_queue() by quiesce queue in next patch.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20231009093324.957829-5-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>