libceph: change how osd_op_reply message size is calculated

For a message pool message, preallocate a page, just like we do for
osd_op.  For a normal message, take ceph_object_id into account and
don't bother subtracting CEPH_OSD_SLAB_OPS ceph_osd_ops.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>