ipv6: exthdrs: Replace opencoded swap() implementation

Get a coccinelle warning as follows:
net/ipv6/exthdrs.c:800:29-30: WARNING opportunity for swap()

Use swap() to replace opencoded implementation.
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230807020947.1991716-1-william.xuanziyang@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/exthdrs.c

@@ -648,7 +648,6 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
 	struct inet6_dev *idev = __in6_dev_get(skb->dev);
 	struct inet6_skb_parm *opt = IP6CB(skb);
 	struct in6_addr *addr = NULL;
-	struct in6_addr daddr;
 	int n, i;
 	struct ipv6_rt_hdr *hdr;
 	struct rt0_hdr *rthdr;
@@ -796,9 +795,7 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
 		return -1;
 	}
 
-	daddr = *addr;
-	*addr = ipv6_hdr(skb)->daddr;
-	ipv6_hdr(skb)->daddr = daddr;
+	swap(*addr, ipv6_hdr(skb)->daddr);
 
 	ip6_route_input(skb);
 	if (skb_dst(skb)->error) {