RDMA/rxe: Remove unused pkt->offset

The pkt->offset field is never used except to assign it to 0.  But it adds
lots of unneeded code. This patch removes the field and related code. This
causes a measurable improvement in performance.

Link: https://lore.kernel.org/r/20210211210455.3274-1-rpearson@hpe.com


Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>