For a TID RDMA WRITE request, a QP on the responder side could be put into
a queue when a hardware flow is not available. A RNR NAK will be returned
to the requester with a RNR timeout value based on the position of the QP
in the queue. The tid_rdma_flow_wt variable is used to calculate the
timeout value and is determined by using a MTU of 4096 at the module
loading time. This could reduce the timeout value by half from the desired
value, leading to excessive RNR retries.

This patch fixes the issue by calculating the flow weight with the real
MTU assigned to the QP.

Fixes: 07b92370 ("IB/hfi1: Add functions to receive TID RDMA WRITE request")
Link: https://lore.kernel.org/r/20191025195836.106825.77769.stgit@awfm-01.aw.intel.com
Cc: <stable@vger.kernel.org>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>