FROMGIT: block/mq-deadline: Add several comments

Make the code easier to read by adding more comments.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
BUG: 187357408
Change-Id: If62eb600614d2883d72ee3bd7e7859ae66b24512
(cherry picked from commit 16c3afdb127bbff7d3552e076e568281765674b7 git://git.kernel.dk/linux-block/

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