blk-throttle: collapse throtl_dispatch() into the work function

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