ipv6: ah6: use swap() to make code cleaner

Use the macro 'swap()' defined in 'include/linux/minmax.h' to avoid
opencoding it.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yao Jing <yao.jing2@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>