netfilter: ipset: "Directory not empty" error message

When an entry flagged with "nomatch" was tested by ipset, it
returned the error message "Kernel error received:
Directory not empty" instead of "<element> is NOT in set <setname>"
(reported by John Brendler).

The internal error code was not properly transformed before returning
to userspace, fixed.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>