RDMA/CMA: add rdma_iw_cm_id() and rdma_res_to_id() helpers

Add a helper function for iwarp drivers to be able to map an
rdma_cm_id to an iw_cm_id.  This is useful for dumping driver specific
NLDEV/RESTRACK connection state.

Add a helper to return the rdma_cm_id pointer from the rdma_restack
pointer.  This is needed for rdma drivers to map a res entry back to
the public rdma_cm_id struct.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>