RDMA/rxe: Return errors for add index and key

Modify rxe_add_index() and rxe_add_key() to return an error if the index
or key is aleady present in the pool.  Currently they print a warning and
silently fail with bad consequences to the caller.

Link: https://lore.kernel.org/r/20210608042552.33275-3-rpearsonhpe@gmail.com


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