ceph: limit osd read size to CEPH_MSG_MAX_DATA_LEN

libceph returns -EIO when read size > CEPH_MSG_MAX_DATA_LEN.

Link: http://tracker.ceph.com/issues/20528
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>