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>