rhashtable: Remove obj_raw_hashfn

Now that the only caller of obj_raw_hashfn is head_hashfn, we can
simply kill it and fold it into the latter.

This patch also moves the common shift from head_hashfn/key_hashfn
into rht_bucket_index.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>