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>