RDMA/rxe: Create AH index and return to user space

Make changes to rdma_user_rxe.h to allow indexing AH objects, passing the
index in UD send WRs to the driver and returning the index to the rxe
provider.

Modify rxe_create_ah() to add an index to AH when created and if called
from a new user provider return it to user space. If called from an old
provider mark the AH as not having a useful index.  Modify rxe_destroy_ah
to drop the index before deleting the object.

Link: https://lore.kernel.org/r/20211007204051.10086-4-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>