IB/hfi1: Initialize bth1 in 16B rc ack builder

IB/hfi1: Initialize bth1 in 16B rc ack builder

[ Upstream commit 8935780b9f7bbad0f2eb5dd43f61ba7b509a1575 ]

It is possible the bth1 variable could be used uninitialized so going
ahead and giving it a default value.

Otherwise we leak stack memory to the network.

Fixes: 5b6cabb0db77 ("IB/hfi1: Add 16B RC/UC support")
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>