block: optimise blk_flush_plug_list

Don't call flush_plug_callbacks if there are no plug callbacks.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
[hch: split from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211020144119.142582-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>