selftests: forwarding: pedit_dsfield: Add pedit munge ip6 dsfield

Extend the pedit_dsfield forwarding selftest with coverage of "pedit ex
munge ip6 dsfield set".

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>