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

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CVE-2023-53292 blk-mq: fix NULL dereference on q->elevator in blk_mq_elv_switch_none

In the Linux kernel, the following vulnerability has been resolved: blk-mq: fix NULL dereference on q-elevator in blkmqelvswitchnone After grabbing q-sysfslock, q-elevator may become NULL because of elevator switch. Fix the NULL dereference on q-elevator by checking it with lock...