bpf: fix null ptr access caused by bpf_prog_test_run_skb

this is a temporary fix
bug-url: https://lore.kernel.org/all/de98e423-a113-4a11-853d-9706cbc07e37@hust.edu.cn/

Signed-off-by: xulang <xulang@uniontech.com>

Author: xu-lang

net/core/lwt_bpf.c

@@ -613,9 +613,12 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 
 	if (ingress)
 		err = skb_cow_head(skb, len + skb->mac_len);
-	else
+	else {
+		if (unlikely(!skb_dst(skb)))
+			return -EINVAL;
 		err = skb_cow_head(skb,
 				   len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
+	}
 	if (unlikely(err))
 		return err;