block: add bio_for_each_bvec_all()

An upcoming Btrfs fix needs to know the original size of a non-cloned
bios. Rather than accessing the bvec table directly, let's add a
bio_for_each_bvec_all() accessor.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>