xfs: fix big endian build

Commit 0fd7275cc42ab734eaa1a2c747e65479bd1e42af ("xfs: fix gcc 4.6
set but not read and unused statement warnings") failed to convert
some code inside XFS_NATIVE_HOST (big endian host code only) and
hence fails to build on such machines. Fix it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>