x86: don't destroy %rbp on kernel-mode faults

From the code:

    "B stepping K8s sometimes report an truncated RIP for IRET exceptions
    returning to compat mode. Check for these here too."

The code then proceeds to truncate the upper 32 bits of %rbp. This means
that when do_page_fault() is finally called, its prologue,

    do_page_fault:
        push %rbp
        movl %rsp, %rbp

will put the truncated base pointer on the stack. This means that the
stack tracer will not be able to follow the base-pointer changes and
will see all subsequent stack frames as unreliable.

This patch changes the code to use a different register (%rcx) for the
checking and leaves %rbp untouched.
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -926,11 +926,11 @@ error_kernelspace:
 	   iret run with kernel gs again, so don't set the user space flag.
 	   B stepping K8s sometimes report an truncated RIP for IRET 
 	   exceptions returning to compat mode. Check for these here too. */
-	leaq irq_return(%rip),%rbp
-	cmpq %rbp,RIP(%rsp) 
+	leaq irq_return(%rip),%rcx
+	cmpq %rcx,RIP(%rsp)
 	je   error_swapgs
-	movl %ebp,%ebp	/* zero extend */
-	cmpq %rbp,RIP(%rsp) 
+	movl %ecx,%ecx	/* zero extend */
+	cmpq %rcx,RIP(%rsp)
 	je   error_swapgs
 	cmpq $gs_change,RIP(%rsp)
         je   error_swapgs