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: NMinfei Huang <mnfhuang@gmail.com>
Signed-off-by: NYan, Zheng <zyan@redhat.com>

fs/ceph/addr.c

@@ -1149,7 +1149,6 @@ static int ceph_write_begin(struct file *file, struct address_space *mapping,
 		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);