RDMA/rxe: Change AH objects to indexed

Make changes to rxe_param.h and rxe_pool.c to allow indexing of AH
objects. Valid indices are non-zero so older providers can be detected.

Link: https://lore.kernel.org/r/20211007204051.10086-3-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>