libceph: support the RADOS copy-from operation

Add support for performing remote object copies using the 'copy-from'
operation.

[ Add COPY_FROM to get_num_data_items(). ]

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>