kthread: Set *newtdb earlier in kthread_add1()

[markj]

Summary

syzbot reported a boot-time crash in g_event_procbody(), a page fault
when dereferencing g_event_td. g_event_td is initialized by the
kproc_kthread_add() call which creates the GEOM event thread:

kproc_kthread_add(g_event_procbody, NULL, &g_proc, &g_event_td,
    RFHIGHPID, 0, "geom", "g_event");

The crash happened just once; I suspect that the caller of
kproc_kthread_add() was preempted after adding the new thread to the
scheduler, and before setting *newtdp, which is equal to g_event_td.

Fix the problem simply by initializing *newtdp earlier. I see no harm
in that, and it matches kproc_create1(). The scheduler provides
sufficient synchronization to ensure that the store is visible to the
new thread, wherever it happens to run.

MFC after: 1 week
Reported by: [e-mail]