x86-64: Add prototype for old_rsp to a header file

So far this has only been used in process_64.c, but the x32 code will
need it in additional code.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/include/asm/processor.h

@@ -948,6 +948,12 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
 
 #define task_pt_regs(tsk)	((struct pt_regs *)(tsk)->thread.sp0 - 1)
 extern unsigned long KSTK_ESP(struct task_struct *task);
+
+/*
+ * User space RSP while inside the SYSCALL fast path
+ */
+DECLARE_PER_CPU(unsigned long, old_rsp);
+
 #endif /* CONFIG_X86_64 */
 
 extern void start_thread(struct pt_regs *regs, unsigned long new_ip,