block: Add bio_advance()

This is prep work for immutable bio vecs; we first want to centralize
where bvecs are modified.

Next two patches convert some existing code to use this function.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>

diff --cc fs/bio.c
Simple merge

diff --cc include/linux/bio.h
Simple merge

diff --cc include/linux/blk_types.h
Simple merge