null_blk: create a helper for throttling

This patch creates a helper for handling throttling code in the
null_handle_cmd().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>