Revert "selftests/mm: replace atomic_bool with pthread_barrier_t"

Revert "selftests/mm: replace atomic_bool with pthread_barrier_t"

This reverts commit e61ef21e27e8deed8c474e9f47f4aa7bc37e138c.

uffd_poll_thread may be called by other tests that do not initialize the
pthread_barrier, so this approach is not correct.  This will revert to
using atomic_bool instead.

Link: https://lkml.kernel.org/r/20241018171734.2315053-3-edliaw@google.com
Fixes: e61ef21e27e8 ("selftests/mm: replace atomic_bool with pthread_barrier_t")
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>