sh: Fix kernel thread stack corruption with preempt.

When I run a preemptive kernel-2.6.20 for SH7780, a created
kthread(pdflush) can not exit by do_exit() in kernel_thread_helper. I
think that the created kthread should have a room for 'struct pt_regs'
space on the stack top, because __switch_to() will refer to the space as
follows using 'regs = task_pt_regs(prev)' and next condition may be true.
Signed-off-by: NHideo Saito <saito@densan.co.jp>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/kernel/process.c

@@ -250,8 +250,7 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long usp,
 		childregs->regs[15] = usp;
 		ti->addr_limit = USER_DS;
 	} else {
-		childregs->regs[15] = (unsigned long)task_stack_page(p) +
-							THREAD_SIZE;
+		childregs->regs[15] = (unsigned long)childregs;
 		ti->addr_limit = KERNEL_DS;
 	}