vti6: Change minimum MTU to IPV4_MIN_MTU, vti6 can carry IPv4 too

A vti6 interface can carry IPv4 as well, so it makes no sense to
enforce a minimum MTU of IPV6_MIN_MTU.

If the user sets an MTU below IPV6_MIN_MTU, IPv6 will be
disabled on the interface, courtesy of addrconf_notify().
Reported-by: Xin Long <lucien.xin@gmail.com>
Fixes: b96f9afe ("ipv4/6: use core net MTU range checking")
Fixes: c6741fbe ("vti6: Properly adjust vti6 MTU from MTU of lower device")
Fixes: 53c81e95 ("ip6_vti: adjust vti mtu according to mtu of lower device")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/ip6_vti.c

@@ -669,7 +669,7 @@ static void vti6_link_config(struct ip6_tnl *t, bool keep_mtu)
 	else
 		mtu = ETH_DATA_LEN - LL_MAX_HEADER - sizeof(struct ipv6hdr);
 
-	dev->mtu = max_t(int, mtu, IPV6_MIN_MTU);
+	dev->mtu = max_t(int, mtu, IPV4_MIN_MTU);
 }
 
 /**
@@ -881,7 +881,7 @@ static void vti6_dev_setup(struct net_device *dev)
 	dev->priv_destructor = vti6_dev_free;
 
 	dev->type = ARPHRD_TUNNEL6;
-	dev->min_mtu = IPV6_MIN_MTU;
+	dev->min_mtu = IPV4_MIN_MTU;
 	dev->max_mtu = IP_MAX_MTU - sizeof(struct ipv6hdr);
 	dev->flags |= IFF_NOARP;
 	dev->addr_len = sizeof(struct in6_addr);