ceph: limit osd write size

OSD has a configurable limitation of max write size. OSD return
error if write request size is larger than the limitation. For now,
set max write size to CEPH_MSG_MAX_DATA_LEN. It should be small
enough.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>