selftests: pmtu: add basic IPv4 and IPv6 PMTU tests

selftests: pmtu: add basic IPv4 and IPv6 PMTU tests

Commit 2666139e1827 ("selftests: net: Introduce first PMTU test") and
follow-ups introduced some PMTU tests, but they all rely on tunneling,
and, particularly, on VTI.

These new tests use simple routing to exercise the generation and
update of PMTU exceptions in IPv4 and IPv6.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>