s390/vmem: 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.

./arch/s390/mm/vmem.c:680:10-11: WARNING opportunity for swap().

[hca@linux.ibm.com: get rid of all temp variables]
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3786Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230117060223.58583-1-jiapeng.chong@linux.alibaba.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/vmem.c

@@ -672,13 +672,7 @@ static int __init memblock_region_cmp(const void *a, const void *b)
 
 static void __init memblock_region_swap(void *a, void *b, int size)
 {
-	struct memblock_region *r1 = a;
-	struct memblock_region *r2 = b;
-	struct memblock_region swap;
-
-	swap = *r1;
-	*r1 = *r2;
-	*r2 = swap;
+	swap(*(struct memblock_region *)a, *(struct memblock_region *)b);
 }
 
 /*