Introduce an rq-qos policy that assigns an I/O priority to requests based
on blk-cgroup configuration settings. This policy has the following
advantages over the ioprio_set() system call:
- This policy is cgroup based so it has all the advantages of cgroups.
- While ioprio_set() does not affect page cache writeback I/O, this rq-qos
controller affects page cache writeback I/O for filesystems that support
assiociating a cgroup with writeback I/O. See also
Documentation/admin-guide/cgroup-v2.rst.

Cc: Damien Le Moal
Cc: Hannes Reinecke
Cc: Christoph Hellwig
Cc: Ming Lei
Cc: Johannes Thumshirn
Cc: Himanshu Madhani
Signed-off-by: Bart Van Assche
Link: https://lore.kernel.org/r/20210618004456.7280-5-bvanassche@acm.org
Signed-off-by: Jens Axboe