RDMA/cma: Export cma_get_service_id()

Allow the rdma_ucm to query the IB service ID formed or allocated by
the rdma_cm by exporting the cma_get_service_id() functionality.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>