ip_tunnel: Add missing validation of encap type to ip_tunnel_encap_setup()

The encap->type comes straight from Netlink. Validate it against
max supported encap types just like ip_encap_hlen() already does.

Fixes: a8c5f9 ("ip_tunnel: Ops registration for secondary encap (fou, gue)")
Signed-off-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -573,6 +573,9 @@ int ip_tunnel_encap(struct sk_buff *skb, struct ip_tunnel *t,
 	if (t->encap.type == TUNNEL_ENCAP_NONE)
 		return 0;
 
+	if (t->encap.type >= MAX_IPTUN_ENCAP_OPS)
+		return -EINVAL;
+
 	rcu_read_lock();
 	ops = rcu_dereference(iptun_encaps[t->encap.type]);
 	if (likely(ops && ops->build_header))