block: move blk_get_flush_queue to blk-flush.c

blk_get_flush_queue is only used in blk-flush.c, so move it there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211123185312.1432157-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>