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

With this change, userfaultfd fails to build with undefined reference
swap() error:

  userfaultfd.c: In function `userfaultfd_stress':
  userfaultfd.c:1530:17: warning: implicit declaration of function `swap'; did you mean `swab'? [-Wimplicit-function-declaration]
   1530 |                 swap(area_src, area_dst);
        |                 ^~~~
        |                 swab
  /usr/bin/ld: /tmp/ccDGOAdV.o: in function `userfaultfd_stress':
  userfaultfd.c:(.text+0x549e): undefined reference to `swap'
  /usr/bin/ld: userfaultfd.c:(.text+0x54bc): undefined reference to `swap'
  collect2: error: ld returned 1 exit status

Revert the commit to fix the problem.

Link: https://lkml.kernel.org/r/20220202003340.87195-1-skhan@linuxfoundation.org
Fixes: 2c769ed7

 ("tools/testing/selftests/vm/userfaultfd.c: use swap() to make code cleaner")
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Minghao Chi <chi.minghao@zte.com.cn>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>