block: move a few merge helpers out of <linux/blkdev.h>

These are block-layer internal helpers, so move them to block/blk.h and
block/blk-merge.c.  Also update a comment a bit to use better grammar.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210920123328.1399408-16-hch@lst.de


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