[PATCH] cfq-iosched: Kill O(N) runtime of cfq_resort_rr_list()

Currently it scales with number of processes in that priority group,
which is potentially not very nice as it's called quite often.
Basically we always need to do tail inserts, except for the case of a
new process. So just mark/detect a queue as such.

Signed-off-by: Jens Axboe <axboe@suse.de>