xfs: remove iomap_delta

The iomap_delta field in struct xfs_iomap just contains the
difference between the offset passed to xfs_iomap and the
iomap_offset.  Just calculate it in the only caller that cares.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>