ip6gretap: Fix MTU to allow for Ethernet header

When we were creating an ip6gretap interface the MTU was about 6 bytes
short of what was needed.  It turns out we were not taking the Ethernet
header into account and as a result we were eating into the 8 bytes
reserved for the encap limit.
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -987,6 +987,8 @@ static void ip6gre_tnl_link_config(struct ip6_tnl *t, int set_mtu)
 				dev->mtu = rt->dst.dev->mtu - addend;
 				if (!(t->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
 					dev->mtu -= 8;
+				if (dev->type == ARPHRD_ETHER)
+					dev->mtu -= ETH_HLEN;
 
 				if (dev->mtu < IPV6_MIN_MTU)
 					dev->mtu = IPV6_MIN_MTU;