IB/usnic: Push all forwarding state to usnic_fwd.[hc]

Push all of the usnic device forwarding state - such as mtu, mac - to
usnic_fwd_dev.  Furthermore, usnic_fwd.h exposes a improved interface
for rest of the usnic code.  The primary improvement is that
usnic_fwd.h's flow management interface takes in high-level *filter*
and *action* structures now, instead of low-level paramaters such as
vnic_idx, rq_idx.

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