btrfs: factor out a btrfs_add_compressed_bio_pages helper

Factor out a common helper to add the compressed_bio pages to the
bio that is shared by the compressed read and write path.

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>