ipv6: ICMPV6: Use swap() instead of open coding it

Swap is a function interface that provides exchange function. To avoid
code duplication, we can use swap function.

./net/ipv6/icmp.c:344:25-26: WARNING opportunity for swap().

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3896
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230131063456.76302-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/icmp.c

@@ -332,7 +332,6 @@ static void mip6_addr_swap(struct sk_buff *skb, const struct inet6_skb_parm *opt
 {
 	struct ipv6hdr *iph = ipv6_hdr(skb);
 	struct ipv6_destopt_hao *hao;
-	struct in6_addr tmp;
 	int off;
 
 	if (opt->dsthao) {
@@ -340,9 +339,7 @@ static void mip6_addr_swap(struct sk_buff *skb, const struct inet6_skb_parm *opt
 		if (likely(off >= 0)) {
 			hao = (struct ipv6_destopt_hao *)
 					(skb_network_header(skb) + off);
-			tmp = iph->saddr;
-			iph->saddr = hao->addr;
-			hao->addr = tmp;
+			swap(iph->saddr, hao->addr);
 		}
 	}
 }