fix page fault from show_trace()

From: Jan Beulich <jbeulich@novell.com>

The introduction of call_softirq switching to the interrupt stack several
releases earlier resulted in a problem with the code in show_trace, which
assumes that it can pick the previous stack pointer from the end of the
interrupt stack.

Cc: Andi Kleen <ak@muc.de>
Cc: Arjan van de Ven <arjanv@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/kernel/entry.S
===================================================================
--- linux.orig/arch/x86_64/kernel/entry.S
+++ linux/arch/x86_64/kernel/entry.S
@@ -1045,17 +1045,15 @@ ENTRY(call_debug)
 ENTRY(call_softirq)
 	CFI_STARTPROC
 	movq %gs:pda_irqstackptr,%rax
-	pushq %r15
-	CFI_ADJUST_CFA_OFFSET 8
-	movq %rsp,%r15
-	CFI_DEF_CFA_REGISTER	r15
+	movq %rsp,%rdx
+	CFI_DEF_CFA_REGISTER	rdx
 	incl %gs:pda_irqcount
 	cmove %rax,%rsp
+	pushq %rdx
+	/*todo CFI_DEF_CFA_EXPRESSION ...*/
 	call __do_softirq
-	movq %r15,%rsp
+	popq %rsp
 	CFI_DEF_CFA_REGISTER	rsp
 	decl %gs:pda_irqcount
-	popq %r15
-	CFI_ADJUST_CFA_OFFSET -8
 	ret
 	CFI_ENDPROC