blk-throttle: reorganize throtl_service_queue passed around as argument

blk-throttle: reorganize throtl_service_queue passed around as argument

throtl_service_queue will be the building block of hierarchy support
and will form a tree.  This patch updates its usages as arguments to
reduce confusion.

* When a service queue is used as the parent role - the host of the
  rbtree - use @parent_sq instead of @sq.

* For functions taking both @tg and @parent_sq, reorder them so that
  the order is (@tg, @parent_sq) not the other way around.  This makes
  the code follow the usual convention of specifying the primary
  target of the operation as the first argument.

This patch doesn't make any functional differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>