IB/mthca: Set GRH:HopLimit when building MLX headers

Global CM packets used by rmda_cm were being sent with a GRH:hopLimit
of zero, causing them to be dropped by the router.  The problem is a
missing initialization of the hop_limit field in mthca_read_ah(),
which was called by build_mlx_header() when sending a MAD on QP1.
Signed-off-by: Rolf Manderscheid <rvm@obsidianresearch.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_av.c

@@ -279,6 +279,7 @@ int mthca_read_ah(struct mthca_dev *dev, struct mthca_ah *ah,
 			(be32_to_cpu(ah->av->sl_tclass_flowlabel) >> 20) & 0xff;
 		header->grh.flow_label    =
 			ah->av->sl_tclass_flowlabel & cpu_to_be32(0xfffff);
+		header->grh.hop_limit     = ah->av->hop_limit;
 		ib_get_cached_gid(&dev->ib_dev,
 				  be32_to_cpu(ah->av->port_pd) >> 24,
 				  ah->av->gid_index % dev->limits.gid_table_len,