lguest: cleanup for map_switcher()

We can use alloc_page() instead of get_zeroed_page() and virt_to_page()
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/core.c

@@ -67,12 +67,11 @@ static __init int map_switcher(void)
 	 * so we make sure they're zeroed.
 	 */
 	for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
-		unsigned long addr = get_zeroed_page(GFP_KERNEL);
-		if (!addr) {
+		switcher_page[i] = alloc_page(GFP_KERNEL|__GFP_ZERO);
+		if (!switcher_page[i]) {
 			err = -ENOMEM;
 			goto free_some_pages;
 		}
-		switcher_page[i] = virt_to_page(addr);
 	}
 
 	/*