ceph: Avoid to propagate the invalid page point

The variant pagep will still get the invalid page point, although ceph
fails in function ceph_update_writeable_page.

To fix this issue, Assigne the page to pagep until there is no failure
in function ceph_update_writeable_page.

Signed-off-by: Minfei Huang <mnfhuang@gmail.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>
diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index ce3c773..6dfff0b 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1149,7 +1149,6 @@
 		page = grab_cache_page_write_begin(mapping, index, 0);
 		if (!page)
 			return -ENOMEM;
-		*pagep = page;
 
 		dout("write_begin file %p inode %p page %p %d~%d\n", file,
 		     inode, page, (int)pos, (int)len);