[XFS] Log the agf_length change in xfs_growfs_data_private().

SGI-PV: 963528
SGI-Modid: xfs-linux-melb:xfs-kern:28856a

Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>