net: ipv6: rpl_iptunnel: Fix memory leak in rpl_input

Free the skb before returning from rpl_input when skb_cow_head() fails.
Use a "drop" label and goto instructions.

Fixes: a7a29f9c ("net: ipv6: add rpl sr tunnel")
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240911174557.11536-1-justin.iurman@uliege.beSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/rpl_iptunnel.c

@@ -263,10 +263,8 @@ static int rpl_input(struct sk_buff *skb)
 	rlwt = rpl_lwt_lwtunnel(orig_dst->lwtstate);
 
 	err = rpl_do_srh(skb, rlwt);
-	if (unlikely(err)) {
-		kfree_skb(skb);
-		return err;
-	}
+	if (unlikely(err))
+		goto drop;
 
 	local_bh_disable();
 	dst = dst_cache_get(&rlwt->cache);
@@ -286,9 +284,13 @@ static int rpl_input(struct sk_buff *skb)
 
 	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
 	if (unlikely(err))
-		return err;
+		goto drop;
 
 	return dst_input(skb);
+
+drop:
+	kfree_skb(skb);
+	return err;
 }
 
 static int nla_put_rpl_srh(struct sk_buff *skb, int attrtype,