IB/mad: Enable loopback of DR SMP responses from userspace

The local loopback of an outgoing DR SMP response is limited to those
that originate at the driver specific SMA implementation during the
driver specific process_mad() function.  This patch enables a
returning DR SMP originating in userspace (or elsewhere) to be
delivered to the local managment stack.  In this specific case the
driver process_mad() function does not consume or process the MAD, so
a reponse mad has not be created and the original MAD must manually be
copied to the MAD buffer that is to be handed off to the local agent.

Signed-off-by: Steve Welch <swelch@systemfabricworks.com>
Acked-by: Hal Rosenstock <hal@xsigo.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>