Item cache might free null page on alloc error

The item cache allocates a page and a little tracking struct for each
cached page.  If the page allocation fails it might try to free a null
page pointer, which isn't allowed.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/item.c

@@ -345,7 +345,8 @@ static struct cached_page *alloc_pg(struct super_block *sb, gfp_t gfp)
 	page = alloc_page(GFP_NOFS | gfp);
 	if (!page || !pg) {
 		kfree(pg);
-		__free_page(page);
+		if (page)
+			__free_page(page);
 		return NULL;
 	}