KVM: remove dummy pages

Currently, kvm allocates some pages and use them as error indicators,
it wastes memory and is not good for scalability

Base on Avi's suggestion, we use the error codes instead of these pages
to indicate the error conditions

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index ebae24b..7972278 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -203,8 +203,7 @@
 	if (!work)
 		return -ENOMEM;
 
-	work->page = bad_page;
-	get_page(bad_page);
+	work->page = get_bad_page();
 	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
 
 	spin_lock(&vcpu->async_pf.lock);