libceph: add ceph_osdc_call() single-page helper

Add a convenience function to osd_client to send Ceph OSD
'class' ops. The interface assumes that the request and
reply data each consist of single pages.

Signed-off-by: Douglas Fuller <dfuller@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>