kthread: Set *tdptr earlier in kproc_kthread_add()

[markj]

Summary

See commit ae77041e0714 ("kthread: Set *newtdp earlier in
kthread_add1()") for details. That commit was incomplete since
g_init()'s first call to kproc_kthread_add() will cause
kproc_kthread_add() to take the *procptr == NULL branch, which avoids
kthread_create().

To ensure that the thread pointer is initialized before the thread
starts running, we have to start the kernel process with RFSTOPPED.
We could perhaps go further and use RFSTOPPED only when tdptr != NULL,
but it's probably better to have consistent behaviour.

Reported by: [e-mail]

[olce]

I don't see any more path where the pointer to the new struct thread won't be initialized before the new thread is started, so I think this completely fixes the initial problem. Moreover, there most probably won't be any noticeable performance impact, since kproc_create() in the end calls fork1() with RFSTOPPED, so the only difference is now that the new process will be released only very slightly later.