futex: Set task::futex_state to DEAD right after handling futex exit

Setting task::futex_state in do_exit() is rather arbitrarily placed for no
reason. Move it into the futex code.

Note, this is only done for the exit cleanup as the exec cleanup cannot set
the state to FUTEX_STATE_DEAD because the task struct is still in active
use.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.439511191@linutronix.de

kernel/exit.c

@@ -837,7 +837,6 @@ void __noreturn do_exit(long code)
 	 * Make sure we are holding no locks:
 	 */
 	debug_check_no_locks_held();
-	futex_exit_done(tsk);
 
 	if (tsk->io_context)
 		exit_io_context(tsk);

kernel/futex.c

@@ -3682,6 +3682,7 @@ void futex_exec_release(struct task_struct *tsk)
 void futex_exit_release(struct task_struct *tsk)
 {
 	futex_exec_release(tsk);
+	futex_exit_done(tsk);
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,