selftests: forwarding: lib: Parameterize IGMPv3/MLDv2 generation

In order to generate IGMPv3 and MLDv2 packets on the fly, the
functions that generate these packets need to be able to generate
packets for different groups and different sources. Generating MLDv2
packets further needs the source address of the packet for purposes of
checksum calculation. Add the necessary parameters, and generate the
payload accordingly by dispatching to helpers added in the previous
patches.

Adjust the sole client, bridge_mdb.sh, as well.

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