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

Since commit 82612de1c98e ("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: 82612de1c98e ("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>

tools/testing/selftests/net/pmtu.sh

@@ -334,7 +334,7 @@ test_pmtu_vti4_link_add_mtu() {
 	fail=0
 
 	min=68
-	max=$((65528 - 20))
+	max=$((65535 - 20))
 	# Check invalid values first
 	for v in $((min - 1)) $((max + 1)); do
 		${ns_a} ip link add vti4_a mtu ${v} type vti local ${veth4_a_addr} remote ${veth4_b_addr} key 10 2>/dev/null