blkcg: stop iteration early if root_rl is the only request list

blkcg: stop iteration early if root_rl is the only request list

__blk_queue_next_rl() finds next request list based on blkg_list
while skipping root_blkg in the list.
OTOH, root_rl is special as it may exist even without root_blkg.

Though the later part of the function handles such a case correctly,
exiting early is good for readability of the code.

Signed-off-by: Jun'ichi Nomura <[email protected]>
Cc: Tejun Heo <[email protected]>
Acked-by: Vivek Goyal <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>