netfilter: nf_tables: Introduce nft_hook_find_ops{,_rcu}()

Also a pretty dull wrapper around the hook->ops.dev comparison for now.
Will search the embedded nf_hook_ops list in future. The ugly cast to
eliminate the const qualifier will vanish then, too.

Since this future list will be RCU-protected, also introduce an _rcu()
variant here.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>