netfilter: Create and use nf_hook_state.

Instead of passing a large number of arguments down into the nf_hook()
entry points, create a structure which carries this state down through
the hook processing layers.

This makes is so that if we want to change the types or signatures of
any of these pieces of state, there are less places that need to be
changed.

Signed-off-by: David S. Miller <davem@davemloft.net>