[PATCH] Prepare for page unmapper

Avoid touching the next page in the NMI handler stack validity check.

That way, we can start unmapping pages that aren't in use, without
triggering this case.

arch/i386/kernel/entry.S

@@ -508,6 +508,15 @@ debug_stack_correct:
 ENTRY(nmi)
 	cmpl $sysenter_entry,(%esp)
 	je nmi_stack_fixup
+	pushl %eax
+	movl %esp,%eax
+	/* Do not access memory above the end of our stack page,
+	 * it might not exist.
+	 */
+	andl $0x1fff,%eax
+	cmpl $0x1fec,%eax
+	popl %eax
+	jae nmi_stack_correct
 	cmpl $sysenter_entry,12(%esp)
 	je nmi_debug_stack_check
 nmi_stack_correct: