rhashtable: compact struct rhashtable_params

By using smaller datatypes this (rather large) struct shrinks considerably
(80 -> 48 bytes on x86_64).

As this is embedded in other structs, this also rerduces size of several
others, e.g. cls_fl_head or nft_hash.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>