btrfs: use the bbio file offset in add_ra_bio_pages

struct btrfs_bio now has a file_offset field set up by all submitters.
Use that value combined with the bio size in add_ra_bio_pages to
calculate the last offset in the bio.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>