block: move bio_get_{first,last}_bvec out of bio.h

bio_get_first_bvec and bio_get_last_bvec are only used in blk-merge.c,
so move them there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012161804.991559-8-hch@lst.de


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