libceph: kill ceph_calc_raw_layout()

There is no caller of ceph_calc_raw_layout() outside of libceph, so
there's no need to export from the module.

Furthermore, there is only one caller, in calc_layout(), and it
is not much more than a simple wrapper for that function.

So get rid of ceph_calc_raw_layout() and embed it instead within
calc_layout().

While touching "osd_client.c", get rid of the unnecessary forward
declaration of __send_request().

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>