RDMA/cxgb4: Add support for iWARP Port Mapper user space service

Based on original work by Vipul Pandya.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>

[ Fix htons -> ntohs to make sparse happy.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>