xfs: xfs_iozero can return positive errno

It was missed when we converted everything in XFs to use negative error
numbers, so fix it now. Bug introduced in 3.17 by commit 2451337d ("xfs: global
error sign conversion"), and should go back to stable kernels.

Thanks to Brian Foster for noticing it.

cc: <stable@vger.kernel.org> # 3.17, 3.18, 3.19, 4.0
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>