selftests: pmtu: maximum MTU for vti4 is 2^16-1-20

Since commit 82612de1 ("ip_tunnel: restore binding to ifaces with a
large mtu"), the maximum MTU for vti4 is based on IP_MAX_MTU instead of
the mysterious constant 0xFFF8.  This makes this selftest fail.

Fixes: 82612de1 ("ip_tunnel: restore binding to ifaces with a large mtu")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>