different processes that happen to use the same user address in the
separate processes will now be treated as distinct futexes rather than the
same futex. We can now honor shared futexes properly by mapping them to a
PROCESS_SHARED umtx_key. Private futexes use THREAD_SHARED umtx_key
objects.
In conjunction with: dchagin
Reviewed by: kib
MFC after: 1 week