This patch fixes all the preempt-after-task->state-is-TASK_DEAD problems we
had.  Right now, the moment procfs does a down() that sleeps in
proc_pid_flush() [it could] our TASK_DEAD state is zapped and we might be
back to TASK_RUNNING to and we trigger this assert:

        schedule();
        BUG();
        /* Avoid "noreturn function does return".  */
        for (;;) ;

I have split out TASK_ZOMBIE and TASK_DEAD into a separate p->exit_state
field, to allow the detaching of exit-signal/parent/wait-handling from
descheduling a dead task.  Dead-task freeing is done via PF_DEAD.

Tested the patch on x86 SMP and UP, but all architectures should work
fine.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>