selftests: pmtu: Add pmtu_vti4_link_add_mtu test

This test checks that MTU given on vti link creation is actually
configured, and that tunnel is not created with an invalid MTU
value.

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