KVM: introduce KVM_ERR_PTR_BAD_PAGE

It is used to eliminate the overload of function call and cleanup
the code

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 7972278..56f5533 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -203,7 +203,7 @@
 	if (!work)
 		return -ENOMEM;
 
-	work->page = get_bad_page();
+	work->page = KVM_ERR_PTR_BAD_PAGE;
 	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
 
 	spin_lock(&vcpu->async_pf.lock);