FROMGIT: block/mq-deadline: Add an invariant check

Check a statistics invariant at module unload time. When running
blktests, the invariant is verified every time a request queue is
removed and hence is verified at least once per test.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Niklas Cassel <Niklas.Cassel@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
(cherry picked from commit 5eb9e5202056efdf49f1fdcfd9f9f86b81a207c9 git://git.kernel.dk/linux-block/

 for-5.16 block)
Change-Id: I912a080a13b7c9c5674d56ba525d97424ad72186
Signed-off-by: Bart Van Assche <bvanassche@google.com>