RDS: make rhashtable_params const

Make this const as it is either used during a copy operation or passed
to a const argument of the function rhltable_init

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>