blk-flush: don't run queue for requests bypassing flush

blk_insert_flush() should only insert request since run queue always
follows it.

In case of bypassing flush, we don't need to run queue because every
blk_insert_flush() follows one run queue.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>