selftests/mm: fix ARM related issue with fork after pthread_create

selftests/mm: fix ARM related issue with fork after pthread_create

commit 8c864371b2a15a23ce35aa7e2bd241baaad6fbe8 upstream.

Following issue was observed while running the uffd-unit-tests selftest
on ARM devices. On x86_64 no issues were detected:

pthread_create followed by fork caused deadlock in certain cases wherein
fork required some work to be completed by the created thread.  Used
synchronization to ensure that created thread's start function has started
before invoking fork.

[edliaw@google.com: refactored to use atomic_bool]
Link: https://lkml.kernel.org/r/20240325194100.775052-1-edliaw@google.com
Fixes: 760aee0b71e3 ("selftests/mm: add tests for RO pinning vs fork()")
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
Signed-off-by: Edward Liaw <edliaw@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>