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 681696862bc18
("selftests: vm: remove dependecy from internal kernel macros")
It has been tested with gcc (Debian 8.3.0-6) 8.3.0.
Signed-off-by: Guo Zhengkui <guozhengkui@xxxxxxxx>