RDMA/rxe: Remove references to ib_device and pool

rxe_pool.c takes references to the pool and ib_device structs for each
object allocated and also keeps an atomic num_elem count in each
pool. This is more work than is needed. Pool allocation is only called
from verbs APIs which already have references to ib_device and pools are
only diasbled when the driver is removed so no protection of the pool
addresses are needed. The elem count is used to warn if elements are still
present in a pool when it is cleaned up which is useful.

This patch eliminates the references to the ib_device and pool structs.

Link: https://lore.kernel.org/r/20210125211641.2694-5-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>