userfaultfd/selftests: use swap() instead of open coding it

Address the following coccicheck warning:

tools/testing/selftests/vm/userfaultfd.c:1536:21-22: WARNING opportunity
for swap().
tools/testing/selftests/vm/userfaultfd.c:1540:33-34: WARNING opportunity
for swap().

by using swap() for the swapping of variable values and drop
`tmp_area` that is not needed any more.

`swap()` macro in userfaultfd.c is introduced in commit 68169686
("selftests: vm: remove dependecy from internal kernel macros")

It has been tested with gcc (Debian 8.3.0-6) 8.3.0.

Link: https://lkml.kernel.org/r/20220407123141.4998-1-guozhengkui@vivo.comSigned-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -1422,7 +1422,6 @@ static void userfaultfd_pagemap_test(unsigned int test_pgsize)
 static int userfaultfd_stress(void)
 {
 	void *area;
-	char *tmp_area;
 	unsigned long nr;
 	struct uffdio_register uffdio_register;
 	struct uffd_stats uffd_stats[nr_cpus];
@@ -1533,13 +1532,9 @@ static int userfaultfd_stress(void)
 					    count_verify[nr], nr);
 
 		/* prepare next bounce */
-		tmp_area = area_src;
-		area_src = area_dst;
-		area_dst = tmp_area;
+		swap(area_src, area_dst);
 
-		tmp_area = area_src_alias;
-		area_src_alias = area_dst_alias;
-		area_dst_alias = tmp_area;
+		swap(area_src_alias, area_dst_alias);
 
 		uffd_stats_report(uffd_stats, nr_cpus);
 	}