x86, idle: Add memory barriers around clflush in mwait_play_dead()

For consistency with mwait_idle_with_hints().  Not sure they help, but
they really won't hurt...

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Link: http://lkml.kernel.org/r/CA%2B55aFzGxcML7j8CEvQPYzh0W81uVoAAVmGctMOUZ7CZ1yYd2A@mail.gmail.com

arch/x86/kernel/smpboot.c

@@ -1417,7 +1417,9 @@ static inline void mwait_play_dead(void)
 		 * The WBINVD is insufficient due to the spurious-wakeup
 		 * case where we return around the loop.
 		 */
+		mb();
 		clflush(mwait_ptr);
+		mb();
 		__monitor(mwait_ptr, 0, 0);
 		mb();
 		__mwait(eax, 0);