selftests: pmtu.sh: Add tests for UDP tunnels handled by Open vSwitch

The new tests check that IP and IPv6 packets exceeding the local PMTU
estimate, forwarded by an Open vSwitch instance from another node,
result in the correct route exceptions being created, and that
communication with end-to-end fragmentation, over GENEVE and VXLAN
Open vSwitch ports, is now possible as a result of PMTU discovery.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>