IB/hfi1: Error path MAD response size is incorrect

If a MAD packet has incorrect header information, the logic uses the reply
path to report the error.  The reply path expects *resp_len to be set
prior to return.  Unfortunately, *resp_len is set to 0 for this path.
This causes an incorrect response packet.

Fix by ensuring that the *resp_len is defaulted to the incoming packet
size (wc->bytes_len - sizeof(GRH)).
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/hfi1/mad.c

 /*
- * Copyright(c) 2015-2017 Intel Corporation.
+ * Copyright(c) 2015-2018 Intel Corporation.
  *
  * This file is provided under a dual BSD/GPLv2 license.  When using or
  * redistributing this file, you may do so under either license.
@@ -4836,7 +4836,7 @@ static int hfi1_process_opa_mad(struct ib_device *ibdev, int mad_flags,
 	int ret;
 	int pkey_idx;
 	int local_mad = 0;
-	u32 resp_len = 0;
+	u32 resp_len = in_wc->byte_len - sizeof(*in_grh);
 	struct hfi1_ibport *ibp = to_iport(ibdev, port);
 
 	pkey_idx = hfi1_lookup_pkey_idx(ibp, LIM_MGMT_P_KEY);