blkio: Determine async workload length based on total number of queues
o Async queues are not per group. Instead these are system wide and maintained
in root group. Hence their workload slice length should be calculated
based on total number of queues in the system and not just queues in the
root group.
o As root group's default weight is 1000, make sure to charge async queue
more in terms of vtime so that it does not get more time on disk because
root group has higher weight.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>