futex/pi: Plug private futex exec() race

In the Linux kernel, the following vulnerability has been resolved:

futex/pi: Plug private futex exec() race

The check for private futexes whether the waiter's mm, which is stored in
the futex_key and copied into the pi_state, is the same as the owner's mm
is not sufficient for exec(). exec() has a gap where the mm check fails to
give the correct answer:

exec()
...
exec_release_mm()
futex_exec_release()
tsk::futex::exit_state = EXITING;
cleanup_robust_list();
1) tsk::futex::exit_state = OK;
...
old_mm = tsk::mm;
2) tsk::mm = ->mm;

Between #1 and #2 the check for the mm is wrong as that mm is about to be
swapped out and eventually freed.

Plug this gap by:

1) Setting tsk::futex::exit_state to FUTEX_STATE_DEAD in
futex_exec_release()

2) Setting tsk::futex::exit_state to FUTEX_STATE_OK after
the mm has been switched.

From a futex point of view the task is dead after it finished the robust
list cleanup up to the point where it sets the state to OK again.