selftests/mm: optionally pass duration to transhuge-stress

Until now, transhuge-stress runs until its explicitly killed, so when
invoked by run_kselftest.sh, it would run until the test timeout, then it
would be killed and the test would be marked as failed.

Add a new, optional command line parameter that allows the user to specify
the duration in seconds that the program should run.  The program exits
after this duration with a success (0) exit code.  If the argument is
omitted the old behacvior remains.

On it's own, this doesn't quite solve our problem because run_kselftest.sh
does not allow passing parameters to the program under test.  But we will
shortly move this to run_vmtests.sh, which does allow parameter passing.

Link: https://lkml.kernel.org/r/20230724082522.1202616-8-ryan.roberts@arm.com


Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>