RDMA/rxe: Replace ah->pd by ah->ibah.pd

The pd field in struct rxe_ah is redundant with the pd field in the
rdma-core's ib_ah. Eliminate the pd field in rxe_ah and add an inline to
extract the pd from the ibah field.

Link: https://lore.kernel.org/r/20211007204051.10086-5-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>