x86/irq/64: Update stale comment

Commit e6401c1 ("x86/irq/64: Split the IRQ stack into its own pages")
missed to update one piece of comment as it did to its peer in Xen, which
will confuse people who still need to read comment.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190719081635.26528-1-caoj.fnst@cn.fujitsu.com

arch/x86/kernel/head_64.S

@@ -193,10 +193,10 @@ ENTRY(secondary_startup_64)
 
 	/* Set up %gs.
 	 *
-	 * The base of %gs always points to the bottom of the irqstack
-	 * union.  If the stack protector canary is enabled, it is
-	 * located at %gs:40.  Note that, on SMP, the boot cpu uses
-	 * init data section till per cpu areas are set up.
+	 * The base of %gs always points to fixed_percpu_data. If the
+	 * stack protector canary is enabled, it is located at %gs:40.
+	 * Note that, on SMP, the boot cpu uses init data section until
+	 * the per cpu areas are set up.
 	 */
 	movl	$MSR_GS_BASE,%ecx
 	movl	initial_gs(%rip),%eax