selftests: pmtu: Add pmtu_vti6_link_change_mtu test

This test checks that MTU configured from userspace is used on
link creation and changes, and that when it's not passed from
userspace, it's calculated properly from the MTU of the lower
layer.

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