xfs: fix non-debug build warnings

There seem to be a couple of new set-but-unused build warnings
that gcc 4.9.3 is now warning about. These are not regressions, just
the compiler being more picky.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>