x86_64: Use __BOOT_DS instead_of __KERNEL_DS for safety

In startup_32, the running code still uses the initial GDT
located in setup. Thus, __BOOT_DS is preferred. Currently
__KERNEL_DS is lucky to equal to __BOOT_DS, but this is
not always a safe way.
Signed-off-by: NLans Zhang <lans.zhang2008@gmail.com>
Link: http://lkml.kernel.org/r/51300267.6000008@gmail.comSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/compressed/head_64.S

@@ -52,7 +52,7 @@ ENTRY(startup_32)
 	jnz 1f
 
 	cli
-	movl	$(__KERNEL_DS), %eax
+	movl	$(__BOOT_DS), %eax
 	movl	%eax, %ds
 	movl	%eax, %es
 	movl	%eax, %ss