x86/asm/entry/32: Remove unnecessary optimization in stub32_clone

Really swap arguments #4 and #5 in stub32_clone instead of
"optimizing" it into a move.

Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a
little bit more expensive than MOV. But a cycle or two on an
expensive syscall like clone() is way below noise floor, and
this optimization is simply not worth the obfuscation of logic.

[ There's also ongoing work on the clone() ABI by Josh Triplett
  that will depend on this change later on. ]
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1433339930-20880-2-git-send-email-dvlasenk@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/ia32entry.S

@@ -529,14 +529,13 @@ GLOBAL(\label)
 GLOBAL(stub32_clone)
 	leaq	sys_clone(%rip), %rax
 	/*
-	 * 32-bit clone API is clone(..., int tls_val, int *child_tidptr).
-	 * 64-bit clone API is clone(..., int *child_tidptr, int tls_val).
-	 * Native 64-bit kernel's sys_clone() implements the latter.
-	 * We need to swap args here. But since tls_val is in fact ignored
-	 * by sys_clone(), we can get away with an assignment
-	 * (arg4 = arg5) instead of a full swap:
+	 * The 32-bit clone ABI is: clone(..., int tls_val, int *child_tidptr).
+	 * The 64-bit clone ABI is: clone(..., int *child_tidptr, int tls_val).
+	 *
+	 * The native 64-bit kernel's sys_clone() implements the latter,
+	 * so we need to swap arguments here before calling it:
 	 */
-	mov	%r8, %rcx
+	xchg	%r8, %rcx
 	jmp	ia32_ptregs_common
 
 	ALIGN