IB/usnic: Port over main.c and verbs.c to the usnic_fwd.h

This patch ports usnic_ib_main.c, usnic_ib_verbs.c and usnic_ib.h
to the new interface of usnic_fwd.h.

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