FROMGIT: blk-mq: clearing flush request reference in tags->rqs[]

Before we free request queue, clearing flush request reference in
tags->rqs[], so that potential UAF can be avoided.

Based on one patch written by David Jeffery.

Tested-by: John Garry <john.garry@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210511152236.763464-5-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Change-Id: I631dbb5138e427246d2e717576fc44727daaa286
Bug: 188199752
(cherry picked from commit 51d4673e57d2613152fdb2ccfe917643472bb218 git://git.kernel.dk/linux-block/

 for-5.14/block)
Signed-off-by: Bart Van Assche <bvanassche@google.com>