kernel - Fix umtx (pthread_mutex etc) race in kernel

kernel - Fix umtx (pthread_mutex etc) race in kernel

* The umtx code could still race a fork despite the wakeup
  due to certain access paths leading into umtx_sleep() only
  issuing reads, thus not triggering a COW operation post-fork.
  This results in a loss of the interlock.

* Fix the issue by having umtx_sleep() fetch the user data
  with a read-modify-write instead of a read.  It now essentially
  does a lock xadd, adding $0 to the memory to force a write
  fault if necessary.

* Fixes 'go' tests and probably other situations that can arise
  when heavily threaded programs use fork/exec.

Reported-by: Tim Darby (+testing)