selftests/mm: fix deadlock for fork after pthread_create on ARM

selftests/mm: fix deadlock for fork after pthread_create on ARM

On Android with arm, there is some synchronization needed to avoid a
deadlock when forking after pthread_create.

Link: https://lkml.kernel.org/r/20241003211716.371786-3-edliaw@google.com
Fixes: cff294582798 ("selftests/mm: extend and rename uffd pagemap test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.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>