libceph: pass length to ceph_osdc_build_request()

The len argument to ceph_osdc_build_request() is set up to be
passed by address, but that function never updates its value
so there's no need to do this.  Tighten up the interface by
passing the length directly.

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