rhashtable: Use head_hashfn instead of obj_raw_hashfn

Now that we don't have cross-table hashes, we no longer need to
keep the entire hash value so all users of obj_raw_hashfn can
use head_hashfn instead.

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>