netfilter: decouple nf_hook_entry and nf_hook_ops

During nfhook traversal we only need a very small subset of
nf_hook_ops members.

We need:
- next element
- hook function to call
- hook function priv argument

Bridge netfilter also needs 'thresh'; can be obtained via ->orig_ops.

nf_hook_entry struct is now 32 bytes on x86_64.

A followup patch will turn the run-time list into an array that only
stores hook functions plus their priv arguments, eliminating the ->next
element.

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>