ip_gre: fix potential memory leak in erspan_rcv

If md is NULL, tun_dst must be freed, otherwise it will cause memory
leak.

Fixes: 1a66a836 ("gre: add collect_md mode to ERSPAN tunnel")
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -313,8 +313,10 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 				return PACKET_REJECT;
 
 			md = ip_tunnel_info_opts(&tun_dst->u.tun_info);
-			if (!md)
+			if (!md) {
+				dst_release((struct dst_entry *)tun_dst);
 				return PACKET_REJECT;
+			}
 
 			memcpy(md, pkt_md, sizeof(*md));
 			md->version = ver;