block: ublk_drv: add common exit handling

Simplify exit handling a bit, and prepare for supporting fused command.

Reviewed-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>