[XFS] Simplify XFS min/max macros.

SGI-PV: 964547
SGI-Modid: xfs-linux-melb:xfs-kern:28945a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nscott@aconex.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>