selftests/mm: fix uffd-stress help information

commit 686a8bb72349("selftests/mm: split uffd tests into uffd-stress and
uffd-unit-tests") split uffd tests into uffd-stress and uffd-unit-tests,
obviously we need to modify the help information synchronously.

Also modify code indentation.

Link: https://lkml.kernel.org/r/tencent_64FC724AC5F05568F41BD1C68058E83CEB05@qq.com
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>