proc: Disallow re-enabling of process itimers during exit

[markj]

Summary

During process exit, it's possible for the exiting thread to send a
signal to its process, via killjobc(). This happens after the itimer is
drained. If itimers are stopped, i.e., P2_ITSTOPPED is set, then
itimer_proc_continue() will resume the callout after it has been
drained.

Fix the problem by simply clearing P2_ITSTOPPED as part of the drain.
Then, a signal received after that point will not re-enable the callout.

Reported by: syzkaller

[kib]

I think this is fine.
But also I suggest to add the check for P_WEXIT/P_KILLED before re-arming the itimer callouts.

[markj]

Check for P_WEXIT in realitexpire_reset_callout() too.