FROMGIT: block/mq-deadline: Track I/O statistics

Track I/O statistics per I/O priority and export these statistics to
debugfs. These statistics help developers of the deadline scheduler.

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
BUG: 187357408
Change-Id: I8e91693dc1d015060737fa2fc15f5f2ebee2530c
(cherry picked from commit 9dc236caf2518c1e434be7a4f8fae60fb0be506a git://git.kernel.dk/linux-block/

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