netfilter: Convert nft_hash to inlined rhashtable

This patch converts nft_hash to the inlined rhashtable interface.

This patch also replaces the call to rhashtable_lookup_compare with
a straight rhashtable_lookup_fast because it's simply doing a memcmp
(in fact nft_hash_lookup already uses memcmp instead of nft_data_cmp).

Furthermore, the compare function is only meant to compare, it is not
supposed to have side-effects.  The current side-effect code can
simply be moved into the nft_hash_get.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>