dummynet: handle IPV6 layer 2 traffic

When pf sends layer 2 traffic into dummynet it still marks IPv6 with
IPFW_ARGS_IPV6 (which dummynet translates to PROTO_V6). That in turn
results in it not matching the 'DIR_IN | PROTO_LAYER2' case, and
triggering the 'bad switch' error message.

Add extra cases for LAYER2 | PROTO_IPV6.

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