IB/mad: Fix ib_post_send_mad() returning 0 with no generate send comp

If ib_post_send_mad() returns 0, the API guarantees that there will be
a callback to send_buf->mad_agent->send_handler() so that the sender
can call ib_free_send_mad().  Otherwise, the ib_mad_send_buf will be
leaked and the mad_agent reference count will never go to zero and the
IB device module cannot be unloaded.  The above can happen without
this patch if process_mad() returns (IB_MAD_RESULT_SUCCESS |
IB_MAD_RESULT_CONSUMED).

If process_mad() returns IB_MAD_RESULT_SUCCESS and there is no agent
registered to receive the mad being sent, handle_outgoing_dr_smp()
returns zero which causes a MAD packet which is at the end of the
directed route to be incorrectly sent on the wire but doesn't cause a
hang since the HCA generates a send completion.

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