powerpc: make fork_idle() take the common "kernel thread" path in copy_thread()

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

arch/powerpc/kernel/process.c

@@ -745,7 +745,7 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 	/* Copy registers */
 	sp -= sizeof(struct pt_regs);
 	childregs = (struct pt_regs *) sp;
-	if (!regs) {
+	if (unlikely(p->flags & PF_KTHREAD)) {
 		struct thread_info *ti = (void *)task_stack_page(p);
 		memset(childregs, 0, sizeof(struct pt_regs));
 		childregs->gpr[1] = sp + sizeof(struct pt_regs);