kthread: Never put_user the set_child_tid address

Kernel threads abuse set_child_tid.  Historically that has been fine
as set_child_tid was initialized after the kernel thread had been
forked.  Unfortunately storing struct kthread in set_child_tid after
the thread is running makes struct kthread being unusable for storing
result codes of the thread.

When set_child_tid is set to struct kthread during fork that results
in schedule_tail writing the thread id to the beggining of struct
kthread (if put_user does not realize it is a kernel address).

Solve this by skipping the put_user for all kthreads.
Reported-by: NNathan Chancellor <nathan@kernel.org>
Link: https://lkml.kernel.org/r/YcNsG0Lp94V13whH@archlinux-ax161Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/sched/core.c

@@ -4908,7 +4908,7 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
 	finish_task_switch(prev);
 	preempt_enable();
 
-	if (current->set_child_tid)
+	if (!(current->flags & PF_KTHREAD) && current->set_child_tid)
 		put_user(task_pid_vnr(current), current->set_child_tid);
 
 	calculate_sigpending();