fou: fix initialization of grc

The grc must be initialize first. There can be a condition where if
fou is NULL, goto out will be executed and grc would be used
uninitialized.

Fixes: 7e41969 ("fou: Fix null-ptr-deref in GRO.")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240906102839.202798-1-usama.anjum@collabora.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Muhammad Usama Anjum

net/ipv4/fou_core.c

@@ -336,11 +336,11 @@ static struct sk_buff *gue_gro_receive(struct sock *sk,
 	struct gro_remcsum grc;
 	u8 proto;
 
+	skb_gro_remcsum_init(&grc);
+
 	if (!fou)
 		goto out;
 
-	skb_gro_remcsum_init(&grc);
-
 	off = skb_gro_offset(skb);
 	len = off + sizeof(*guehdr);