x86, 64-bit: Set data segments to null after switching to 64-bit mode

This prevents kernel threads from inheriting non-null segment
selectors, and causing optimizations in __switch_to() to be
ineffective.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Cc: Tim Blechmann <tim@klingt.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jan Beulich <JBeulich@novell.com>
LKML-Reference: <1259165856-3512-1-git-send-email-brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/head_64.S

@@ -212,8 +212,8 @@ ENTRY(secondary_startup_64)
 	 */
 	lgdt	early_gdt_descr(%rip)
 
-	/* set up data segments. actually 0 would do too */
-	movl $__KERNEL_DS,%eax
+	/* set up data segments */
+	xorl %eax,%eax
 	movl %eax,%ds
 	movl %eax,%ss
 	movl %eax,%es