rhashtable: Allow hashfn to be unset

Since every current rhashtable user uses jhash as their hash
function, the fact that jhash is an inline function causes each
user to generate a copy of its code.

This function provides a solution to this problem by allowing
hashfn to be unset.  In which case rhashtable will automatically
set it to jhash.  Furthermore, if the key length is a multiple
of 4, we will switch over to jhash2.

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>