Fix the check supposed to prevent 'ip' and 'ip-stealth' balancing modes

Fix the check supposed to prevent 'ip' and 'ip-stealth' balancing modes
from leaking the multicast address.

beck@ found the hard way that this made his second CARP master use a
wrong MAC address.

This is part of a bigger diff from Florian Riehm who is currently
working on a proper solution to fix balancing modes.

ok beck@, bluhm@