ip_gre: fix wrong return value of erspan_rcv

If pskb_may_pull return failed, return PACKET_REJECT instead of -ENOMEM.

Fixes: 84e54fe0 ("gre: introduce native tunnel support for ERSPAN")
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -266,7 +266,7 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	len = gre_hdr_len + sizeof(*ershdr);
 
 	if (unlikely(!pskb_may_pull(skb, len)))
-		return -ENOMEM;
+		return PACKET_REJECT;
 
 	iph = ip_hdr(skb);
 	ershdr = (struct erspanhdr *)(skb->data + gre_hdr_len);