locking/ww_mutex/test: Use swap() macro

Fixes the following Coccinelle/coccicheck warning reported by
swap.cocci:

  WARNING opportunity for swap()

Compile-tested only.

[Boqun: Add the report tags from Jiapeng and Abaci Robot [1].]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reported-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=11531
Link: https://lore.kernel.org/r/20241025081455.55089-1-jiapeng.chong@linux.alibaba.com

 [1]
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240731135850.81018-2-thorsten.blum@toblux.com