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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>