IB/qib: Fix double add_timer()

The following panic BUG_ON occurs during qib testing:

    Kernel BUG at include/linux/timer.h:82

    RIP  [<ffffffff881f7109>] :ib_qib:start_timer+0x73/0x89
     RSP <ffffffff80425bd0>
     <0>Kernel panic - not syncing: Fatal exception
     <0>Dumping qib trace buffer from panic
    qib_set_lid INFO: IB0:1 got a lid: 0xf8
    Done dumping qib trace buffer
    BUG: warning at kernel/panic.c:137/panic() (Tainted: G

The flaw is due to a missing state test when processing responses that
results in an add_timer() call when the same timer is already queued.
This code was executing in parallel with a QP destroy on another CPU
that had changed the state to reset, but the missing test caused to
response handling code to run on into the panic.
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_rc.c

@@ -1439,6 +1439,8 @@ static void qib_rc_rcv_resp(struct qib_ibport *ibp,
 	}
 
 	spin_lock_irqsave(&qp->s_lock, flags);
+	if (!(ib_qib_state_ops[qp->state] & QIB_PROCESS_RECV_OK))
+		goto ack_done;
 
 	/* Ignore invalid responses. */
 	if (qib_cmp24(psn, qp->s_next_psn) >= 0)