xfs: refactor xfs_inode_item_format

Split out a function to handle the data and attr fork, as well as a
helper for the really old v1 inodes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>