RDMA/rxe: Make add/drop key/index APIs type safe

Replace 'void *' parameters with 'struct rxe_pool_entry *' and use a macro
to allow:
   rxe_add_index,
   rxe_drop_index,
   rxe_add_key,
   rxe_drop_key and
   rxe_add_to_pool
APIs to be type safe against changing the position of pelem in the
objects.

Link: https://lore.kernel.org/r/20201216231550.27224-6-rpearson@hpe.com


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