xen: handle highmem pages correctly when shrinking a domain

Commit 1058a75f ("xen: actually release
memory when shrinking domain") causes a crash if the page being released
is a highmem page.

If a page is highmem then there is no need to unmap it.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

drivers/xen/balloon.c

@@ -299,12 +299,15 @@ static int decrease_reservation(unsigned long nr_pages)
 
 		scrub_page(page);
 
+		if (!PageHighMem(page)) {
 			ret = HYPERVISOR_update_va_mapping(
 				(unsigned long)__va(pfn << PAGE_SHIFT),
 				__pte_ma(0), 0);
 			BUG_ON(ret);
                 }
 
+	}
+
 	/* Ensure that ballooned highmem pages don't have kmaps. */
 	kmap_flush_unused();
 	flush_tlb_all();