ipv4: ip_gre: Fix set but not used warning in ipgre_err() if IPv4-only

if CONFIG_NET_IPGRE is enabled, but CONFIG_IPV6 is disabled:

    net/ipv4/ip_gre.c: In function ‘ipgre_err’:
    net/ipv4/ip_gre.c:144:22: error: variable ‘data_len’ set but not used [-Werror=unused-but-set-variable]
      144 |         unsigned int data_len = 0;
	  |                      ^~~~~~~~

Fix this by moving all data_len processing inside the IPV6-only section
that uses its result.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501121007.2GofXmh5-lkp@intel.com/
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/d09113cfe2bfaca02f3dddf832fb5f48dd20958b.1738704881.git.geert@linux-m68k.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_gre.c

@@ -141,7 +141,6 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 	const struct iphdr *iph;
 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
-	unsigned int data_len = 0;
 	struct ip_tunnel *t;
 
 	if (tpi->proto == htons(ETH_P_TEB))
@@ -182,18 +181,23 @@ static int ipgre_err(struct sk_buff *skb, u32 info,
 	case ICMP_TIME_EXCEEDED:
 		if (code != ICMP_EXC_TTL)
 			return 0;
-		data_len = icmp_hdr(skb)->un.reserved[1] * 4; /* RFC 4884 4.1 */
 		break;
 
 	case ICMP_REDIRECT:
 		break;
 	}
 
 #if IS_ENABLED(CONFIG_IPV6)
-	if (tpi->proto == htons(ETH_P_IPV6) &&
-	    !ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len,
-					type, data_len))
-		return 0;
+	if (tpi->proto == htons(ETH_P_IPV6)) {
+		unsigned int data_len = 0;
+
+		if (type == ICMP_TIME_EXCEEDED)
+			data_len = icmp_hdr(skb)->un.reserved[1] * 4; /* RFC 4884 4.1 */
+
+		if (!ip6_err_gen_icmpv6_unreach(skb, iph->ihl * 4 + tpi->hdr_len,
+						type, data_len))
+			return 0;
+	}
 #endif
 
 	if (t->parms.iph.daddr == 0 ||