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

This patch is in response to reviewing a patch to the core MAD
processing which fixes loopback of directed route packets to/from user
level MAD agents.  This change enables the core code to work for
ib_ipath by fixing the return code from the ipath process_mad method.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>