blk-mq: fix NULL dereference on q->elevator in blk_mq_elv_switch_none

After grabbing q->sysfs_lock, q->elevator may become NULL because of
elevator switch.

Fix the NULL dereference on q->elevator by checking it with lock.

Reported-by: Guangwu Zhang <guazhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230616132354.415109-1-ming.lei@redhat.com


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