[PATCH nf-next 0/3] netfilter: conntrack: prepare for hashtable merge, take 1

[Florian Westphal <fw@strlen.de>]

This small series prepares for upcoming merge of the per-namespace hash tables
into a single table (or rater, three tables
-- conntrack hash, expect hash and nat bysrc hash).

Arguments for merging it:
- We stop wasting (assuming default size) 250k backing store per namespace
- net namespace is just another part of the connection id, much like
  ip addresses or conntrack zones -- no need to treat it specially
- allows to get rid of the per-netns conntrack slab as well

These patches are first preparations.
We replace per-netns conntrack has generation seqcount by single one.

While at it, this replaces the method used to obtain the hash seed
by the (nowadays) more common get_random_once().