x86-64: Move kernelstack from PDA to per-cpu.

Also clean up PER_CPU_VAR usage in xen-asm_64.S

tj: * remove now unused stack_thread_info()
    * s/kernelstack/kernel_stack/
    * added FIXME comment in xen-asm_64.S

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/arch/x86/include/asm/pda.h b/arch/x86/include/asm/pda.h
index 7209302..4d28ffb 100644
--- a/arch/x86/include/asm/pda.h
+++ b/arch/x86/include/asm/pda.h
@@ -13,7 +13,7 @@
 struct x8664_pda {
 	unsigned long unused1;
 	unsigned long unused2;
-	unsigned long kernelstack;	/* 16 top of kernel stack for current */
+	unsigned long unused3;
 	unsigned long oldrsp;		/* 24 user rsp for system call */
 	int irqcount;			/* 32 Irq nesting counter. Starts -1 */
 	unsigned int unused6;		/* 36 was cpunumber */
@@ -44,6 +44,4 @@
 
 #endif
 
-#define PDA_STACKOFFSET (5*8)
-
 #endif /* _ASM_X86_PDA_H */