tools/testing/selftests/vm/userfaultfd.c: use swap() to make code cleaner

Fix the following coccicheck REVIEW:

 tools/testing/selftests/vm/userfaultfd.c:1531:21-22:use swap() to make code cleaner

Link: https://lkml.kernel.org/r/20211124031632.35317-1-chi.minghao@zte.com.cn


Signed-off-by: chiminghao <chi.minghao@zte.com.cn>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>