selftests: forwarding: lib: Allow list of IPs for IGMPv3/MLDv2

The testsuite that checks for mcast_max_groups functionality will need
to generate IGMP and MLD packets with configurable number of (S,G)
addresses. To that end, further extend igmpv3_is_in_get() and
mldv2_is_in_get() to allow a list of IP addresses instead of one
address.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>