cobalt/thread: Move tcb clearing into __xnthread_init

This both de-duplicates the code and ensures that all fields are zeroed
prior to calling one of the actual tcb initialization functions.
Specifically if host_task is not properly cleaned, we may cause a bug
when using the field earlier, e.g.

general protection fault: 0000 [#1] PREEMPT SMP
[...]
RIP: 0010:[<ffffffff81185a3c>]  [<ffffffff81185a3c>] xnthread_host_pid+0x1c/0x30
[..]
Call Trace:
 [<ffffffff8117c987>] trace_event_raw_event_cobalt_thread_set_current_prio+0x57/0xa0
 [<ffffffff8117f33d>] xnsched_set_effective_priority+0x8d/0xc0
 [<ffffffff8117a1e4>] xnsched_rt_setparam+0x14/0x30
 [<ffffffff8117e700>] xnsched_set_policy+0xc0/0x170
 [<ffffffff81185687>] __xnthread_init+0x317/0x3d0
 [<ffffffff8114a3e8>] ? trace_buffer_unlock_commit+0x58/0x70
 [<ffffffff811857bb>] xnthread_init+0x7b/0x110
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kernel/cobalt/thread.c

@@ -208,6 +208,7 @@ int __xnthread_init(struct xnthread *thread,
 	thread->entry = NULL;
 	thread->cookie = NULL;
 	init_completion(&thread->exited);
+	memset(xnthread_archtcb(thread), 0, sizeof(struct xnarchtcb));
 
 	gravity = flags & XNUSER ? XNTIMER_UGRAVITY : XNTIMER_KGRAVITY;
 	xntimer_init(&thread->rtimer, &nkclock, timeout_handler,
@@ -256,7 +257,6 @@ void xnthread_init_shadow_tcb(struct xnthread *thread)
 	 */
 	__ipipe_share_current(0);
 
-	memset(tcb, 0, sizeof(*tcb));
 	tcb->core.host_task = p;
 	tcb->core.tsp = &p->thread;
 	tcb->core.mm = p->mm;
@@ -275,7 +275,6 @@ void xnthread_init_root_tcb(struct xnthread *thread)
 	struct xnarchtcb *tcb = xnthread_archtcb(thread);
 	struct task_struct *p = current;
 
-	memset(tcb, 0, sizeof(*tcb));
 	tcb->core.host_task = p;
 	tcb->core.tsp = &tcb->core.ts;
 	tcb->core.mm = p->mm;