selftests: pmtu: implement IPIP, SIT and ip6tnl PMTU discovery tests

Add PMTU discovery tests for these encapsulations:

- IPIP
- SIT, mode ip6ip
- ip6tnl, modes ip6ip6 and ipip6

Signed-off-by: Lourdes Pedrajas <lu@pplo.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>