RDMA/rxe: Convert kernel UD post send to use ah_num

Modify ib_post_send for kernel UD sends to put the AH index into the WQE
instead of the address vector.

Link: https://lore.kernel.org/r/20211007204051.10086-7-rpearsonhpe@gmail.com


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