libceph: bump CEPH_MSG_MAX_DATA_LEN (again)

This time for rbd object map.  Object maps are limited in size to
256000000 objects, two bits per object.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>