RDMA/cma: Add support for AF_IB to cma_get_service_id()

cma_get_service_id() forms the service ID based on the port space and
port number of the rdma_cm_id.  Extend the call to support AF_IB,
which contains the service ID directly.  This will be needed to
support any arbitrary SID.

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