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/[email protected]
Signed-off-by: Bob Pearson <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

Author: Bob Pearson