Set types bitmap:ipmac and hash:ipmac check that MAC addresses
are not all zeroes.

Introduce one missing check, and make the remaining ones
consistent, using is_zero_ether_addr() instead of comparing
against an array containing zeroes.

This was already done for hash:mac sets in commit 26c97c5d
("netfilter: ipset: Use is_zero_ether_addr instead of static and
memcmp").
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>