IB/usnic: Add UDP support to usnic_fwd.[hc]

Add *ip field* to *struct usnic_fwd_dev* as well as new *functions* to
manipulate the *ip field.*  Furthermore, add new functions for
programming UDP flows in the forwarding device.

Signed-off-by: Upinder Malhi <umalhi@cisco.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>