blk-throttle: Use helper function to add root throtl group to lists

Use same helper function for root group as we use with dynamically
allocated groups to add it to various lists.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

diff --cc block/blk-throttle.c
Simple merge