BACKPORT: 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>

Bug: 197804811
Change-Id: I9600626e807a4eed546c21be808fabed2a9db9b1
(cherry picked from commit 364b61818f65045479e42e76ed8dd6f051778280)
[ refactored to avoid breaking KMI ]
Signed-off-by: Pradeep P V K <pragalla@codeaurora.org>
Signed-off-by: Todd Kjos <tkjos@google.com>