blk-throttle is about to go through major restructuring to support
hierarchy.  Do cosmetic updates in preparation.

* s/throtl_data->throtl_work/throtl_data->dispatch_work/

* s/blk_throtl_work()/blk_throtl_dispatch_work_fn()/

* Collapse throtl_dispatch() into blk_throtl_dispatch_work_fn()

This patch is purely cosmetic.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NVivek Goyal <vgoyal@redhat.com>