If alloc_percpu() fails, we accidentally return PTR_ERR(NULL), which
means success, but we intended to return -ENOMEM.

Fixes: 225e4635 ('xfs: per-filesystem stats in sysfs')
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NBill O'Donnell <billodo@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>