[XFS] Handle null returned from xfs_vtoi() in xfs_setfilesize().

SGI-PV: 965636
SGI-Modid: xfs-linux-melb:xfs-kern:28777a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Olaf Weber <olaf@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>