erspan: fix invalid erspan version.

ERSPAN only support version 1 and 2.  When packets send to an
erspan device which does not have proper version number set,
drop the packet.  In real case, we observe multicast packets
sent to the erspan pernet device, erspan0, which does not have
erspan version configured.
Reported-by: NGreg Rose <gvrose8192@gmail.com>
Signed-off-by: NWilliam Tu <u9012063@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -722,10 +722,12 @@ static netdev_tx_t erspan_xmit(struct sk_buff *skb,
 		erspan_build_header(skb, ntohl(tunnel->parms.o_key),
 				    tunnel->index,
 				    truncate, true);
-	else
+	else if (tunnel->erspan_ver == 2)
 		erspan_build_header_v2(skb, ntohl(tunnel->parms.o_key),
 				       tunnel->dir, tunnel->hwid,
 				       truncate, true);
+	else
+		goto free_skb;
 
 	tunnel->parms.o_flags &= ~TUNNEL_KEY;
 	__gre_xmit(skb, dev, &tunnel->parms.iph, htons(ETH_P_ERSPAN));

net/ipv6/ip6_gre.c

@@ -979,11 +979,14 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 			erspan_build_header(skb, ntohl(t->parms.o_key),
 					    t->parms.index,
 					    truncate, false);
-		else
+		else if (t->parms.erspan_ver == 2)
 			erspan_build_header_v2(skb, ntohl(t->parms.o_key),
 					       t->parms.dir,
 					       t->parms.hwid,
 					       truncate, false);
+		else
+			goto tx_err;
+
 		fl6.daddr = t->parms.raddr;
 	}