Regression in r352027, ping6: unable to limit access to system.dns service

[Raúl Muñoz - CUSTOS]

Hello.

I've already filled a PR (#240437) but given how busy will be everybody
with 12.1 release and that the problem has just arrived to 12-STABLE
tree ... when I run ping6 on a r352027 build or later, I get:

[....]
# ping6
ping6: unable to limit access to system.dns service: Invalid argument
[....]

Instead of (r351925):

[....]
# ping6
usage: ping6 [-DdfHmnNoqrRtvwW] [-a addrtype] [-b bufsiz] [-c count] [-g
gateway]
             [-h hoplimit] [-I interface] [-i wait] [-l preload] [-P policy]
             [-p pattern] [-S sourceaddr] [-s packetsize] [-x waittime]
             [-X timeout] [hops ...] host
[....]

Cheers,
Raúl.