vhost: Take a reference on the task in struct vhost_task.

vhost_task_create() creates a task and keeps a reference to its
task_struct. That task may exit early via a signal and its task_struct
will be released.
A pending vhost_task_wake() will then attempt to wake the task and
access a task_struct which is no longer there.

Acquire a reference on the task_struct while creating the thread and
release the reference while the struct vhost_task itself is removed.
If the task exits early due to a signal, then the vhost_task_wake() will
still access a valid task_struct. The wake is safe and will be skipped
in this case.

Fixes: f9010dbdce911 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
Reported-by: Sean Christopherson <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Tested-by: Sean Christopherson <[e-mail]>

kernel/vhost_task.c

···
 	 * freeing it below.
 	 */
 	wait_for_completion(&vtsk->exited);
+	put_task_struct(vtsk->task);
 	kfree(vtsk);
 }
 EXPORT_SYMBOL_GPL(vhost_task_stop);
···
 		return ERR_CAST(tsk);
 	}
 
-	vtsk->task = tsk;
+	vtsk->task = get_task_struct(tsk);
 	return vtsk;
 }
 EXPORT_SYMBOL_GPL(vhost_task_create);