x86/xen: fix upper bound of pmd loop in xen_cleanhighmap()

xen_cleanhighmap() is operating on level2_kernel_pgt only. The upper
bound of the loop setting non-kernel-image entries to zero should not
exceed the size of level2_kernel_pgt.
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>

arch/x86/xen/mmu.c

@@ -1113,7 +1113,7 @@ static void __init xen_cleanhighmap(unsigned long vaddr,
 
 	/* NOTE: The loop is more greedy than the cleanup_highmap variant.
 	 * We include the PMD passed in on _both_ boundaries. */
-	for (; vaddr <= vaddr_end && (pmd < (level2_kernel_pgt + PAGE_SIZE));
+	for (; vaddr <= vaddr_end && (pmd < (level2_kernel_pgt + PTRS_PER_PMD));
 			pmd++, vaddr += PMD_SIZE) {
 		if (pmd_none(*pmd))
 			continue;