sh: Fix up restorer in debug_trap exception return path.

There are a few different types of debug trap exceptions, though now
that they are all going through a special jump table, the restorer needs
to be unified as well.

Presently this is falling through the ret_from_fork path, which more or
less does the right thing on SH-3/4 whilst being completely unsuitable on
MMU-less targets.

Ultimately what we want here is a branch through the platform's
restore_all directly, without worrying about the retval being clobbered.
We can accomplish that through a branch to __restore_all directly, so
switch it so we come back from the jump table and branch to the restorer.

This fixes up a recursion in the nommu WARN_ON() path, as well as some
other userspace nastiness where said recursion caused serious stack
corruption.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/entry-common.S

@@ -1,9 +1,6 @@
-/* $Id: entry.S,v 1.37 2004/06/11 13:02:46 doyu Exp $
- *
- *  linux/arch/sh/entry.S
- *
+/* 
  *  Copyright (C) 1999, 2000, 2002  Niibe Yutaka
- *  Copyright (C) 2003  Paul Mundt
+ *  Copyright (C) 2003 - 2008  Paul Mundt
  *
  * This file is subject to the terms and conditions of the GNU General Public
  * License.  See the file "COPYING" in the main directory of this archive
@@ -282,7 +279,9 @@ debug_trap:
 	mov.l	1f, r8
 	add	r0, r8
 	mov.l	@r8, r8
-	jmp	@r8
+	jsr	@r8
+	 nop
+	bra	__restore_all
 	 nop
 
 	.align	2