xfs: Fix a build warning in xfs_aops.c

Fix a build warning that slipped through.  Dave Chinner had posted
an updated version of his patch but the previous version--without
this fix--was what got committed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>