raid5-ppl: 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: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Song Liu <songliubraving@fb.com>