selftests: pmtu: Minimum MTU for vti6 is 68

A vti6 interface can carry IPv4 packets too.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>