RDMA/rxe: Let pools support both keys and indices

Allow both indices and keys to exist for objects in pools.  Previously you
were limited to one or the other.

This is required for later implementing rxe memory windows.

Link: https://lore.kernel.org/r/20201216231550.27224-3-rpearson@hpe.com


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