block: move bio_map_* to blk-map.c

The bio_map_* helpers are just the low-level helpers for the
blk_rq_map_* APIs.  Move them together for better logical grouping,
as no there isn't much overlap with other code in bio.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>