blk-mq: protect q->elevator by ->sysfs_lock in blk_mq_elv_switch_none

elevator can be tore down by sysfs switch interface or disk release, so
hold ->sysfs_lock before referring to q->elevator, then potential
use-after-free can be avoided.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220616014401.817001-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>