netfilter: nft_hash: bug fixes and resizing

netfilter: nft_hash: bug fixes and resizing

The hash set type is very broken and was never meant to be merged in this
state. Missing RCU synchronization on element removal, leaking chain
refcounts when used as a verdict map, races during lookups, a fixed table
size are probably just some of the problems. Luckily it is currently
never chosen by the kernel when the rbtree type is also available.

Rewrite it to be usable.

The new implementation supports automatic hash table resizing using RCU,
based on Paul McKenney's and Josh Triplett's algorithm "Optimized Resizing
For RCU-Protected Hash Tables" described in [1].

Resizing doesn't require a second list head in the elements, it works by
chosing a hash function that remaps elements to a predictable set of buckets,
only resizing by integral factors and

- during expansion: linking new buckets to the old bucket that contains
  elements for any of the new buckets, thereby creating imprecise chains,
  then incrementally seperating the elements until the new buckets only
  contain elements that hash directly to them.

- during shrinking: linking the hash chains of all old buckets that hash
  to the same new bucket to form a single chain.

Expansion requires at most the number of elements in the longest hash chain
grace periods, shrinking requires a single grace period.

Due to the requirement of having hash chains/elements linked to multiple
buckets during resizing, homemade single linked lists are used instead of
the existing list helpers, that don't support this in a clean fashion.
As a side effect, the amount of memory required per element is reduced by
one pointer.

Expansion is triggered when the load factors exceeds 75%, shrinking when
the load factor goes below 30%. Both operations are allowed to fail and
will be retried on the next insertion or removal if their respective
conditions still hold.

[1] http://dl.acm.org/citation.cfm?id=2002181.2002192

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>