lib/test_vmalloc.c: use swap() to make code cleaner

Use swap() in order to make code cleaner.  Issue found by coccinelle.

Link: https://lkml.kernel.org/r/20211028111443.15744-1-deng.changcheng@zte.com.cn


Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>