selftests: pmtu: Add pmtu_vti4_default_mtu test

This test checks that the MTU assigned by default to a vti (IPv4)
interface created on top of veth is simply veth's MTU minus the
length of the encapsulated IPv4 header.

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