[XFS] Mark some lookup tables const. Thanks to Arjan van de Ven for

spotting these.

SGI-PV: 946028
SGI-Modid: xfs-linux-melb:xfs-kern:202617a

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