[XFS] various fixes for xfs_convert_page fix various bogusities in

handling offets  From David Chinner and Christoph Hellwig

SGI-PV: 947118
SGI-Modid: xfs-linux-melb:xfs-kern:203826a

Signed-off-by: Christoph Hellwig <hch@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>