x86/asm/entry/64: Simplify 'old_rsp' usage

Remove all manipulations of PER_CPU(old_rsp) in C code:

 - it is not used on SYSRET return anymore, and system entries
   are atomic, so updating it from the fork and context switch
   paths is pointless.

 - Tweak a few related comments as well: we no longer have a
   "partial stack frame" on entry, ever.

Based on (split out of) patch from Denys Vlasenko.

Originally-from: Denys Vlasenko <dvlasenk@redhat.com>
Tested-by: Borislav Petkov <bp@alien8.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1426599779-8010-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/processor.h

@@ -905,11 +905,6 @@ 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,

arch/x86/kernel/process_64.c

@@ -238,7 +238,6 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 	current->thread.usersp	= new_sp;
 	regs->ip		= new_ip;
 	regs->sp		= new_sp;
-	this_cpu_write(old_rsp, new_sp);
 	regs->cs		= _cs;
 	regs->ss		= _ss;
 	regs->flags		= X86_EFLAGS_IF;
@@ -399,7 +398,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	 * Switch the PDA and FPU contexts.
 	 */
 	prev->usersp = this_cpu_read(old_rsp);
-	this_cpu_write(old_rsp, next->usersp);
 	this_cpu_write(current_task, next_p);
 
 	/*