adding missing rcu_read_unlock in ipxip6_rcv

commit 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
introduced new exit point in  ipxip6_rcv. however rcu_read_unlock is
missing there. this diff is fixing this

v1->v2:
 instead of doing rcu_read_unlock in place, we are going to "drop"
 section (to prevent skb leakage)

Fixes: 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
Signed-off-by: Nikita V. Shirokov <tehnerd@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -904,7 +904,7 @@ static int ipxip6_rcv(struct sk_buff *skb, u8 ipproto,
 		if (t->parms.collect_md) {
 			tun_dst = ipv6_tun_rx_dst(skb, 0, 0, 0);
 			if (!tun_dst)
-				return 0;
+				goto drop;
 		}
 		ret = __ip6_tnl_rcv(t, skb, tpi, tun_dst, dscp_ecn_decapsulate,
 				    log_ecn_error);