IB/hfi1: Get rid of hot path divide

IB/hfi1: Get rid of hot path divide

The pointer math in this statemet does a divide;
	struct hfi1_ipoib_txq *txq = &priv->txqs[napi - priv->tx_napis];

Elminate the divide by embedding the struct napi_strut in the txq and
getting the txq with a container_of() using the newly embedded napi.

Fixes: d99dc602e2a5 ("IB/hfi1: Add functions to transmit datagram ipoib packets")
Link: https://lore.kernel.org/r/20210913132831.131370.3993.stgit@awfm-01.cornelisnetworks.com
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>