IB/hfi1: Mask the path bits with the LMC for 16B RC Acks

16B packets require that the path bits are masked with the LMC.
This mask is done correctly in all 16B header creation but was
left out for the RC Acknowledge.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>