rhashtable: Add rhashtable_lookup_get_insert_fast

Add rhashtable_lookup_get_insert_fast for fixed keys, similar to
rhashtable_lookup_get_insert_key for explicit keys.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>