netfilter: nf_tables: add netlink set API

This patch adds the new netlink API for maintaining nf_tables sets
independently of the ruleset. The API supports the following operations:

- creation of sets
- deletion of sets
- querying of specific sets
- dumping of all sets

- addition of set elements
- removal of set elements
- dumping of all set elements

Sets are identified by name, each table defines an individual namespace.
The name of a set may be allocated automatically, this is mostly useful
in combination with the NFT_SET_ANONYMOUS flag, which destroys a set
automatically once the last reference has been released.

Sets can be marked constant, meaning they're not allowed to change while
linked to a rule. This allows to perform lockless operation for set
types that would otherwise require locking.

Additionally, if the implementation supports it, sets can (as before) be
used as maps, associating a data value with each key (or range), by
specifying the NFT_SET_MAP flag and can be used for interval queries by
specifying the NFT_SET_INTERVAL flag.

Set elements are added and removed incrementally. All element operations
support batching, reducing netlink message and set lookup overhead.

The old "set" and "hash" expressions are replaced by a generic "lookup"
expression, which binds to the specified set. Userspace is not aware
of the actual set implementation used by the kernel anymore, all
configuration options are generic.

Currently the implementation selection logic is largely missing and the
kernel will simply use the first registered implementation supporting the
requested operation. Eventually, the plan is to have userspace supply a
description of the data characteristics and select the implementation
based on expected performance and memory use.

This patch includes the new 'lookup' expression to look up for element
matching in the set.

This patch includes kernel-doc descriptions for this set API and it
also includes the following fixes.

From Patrick McHardy:
* netfilter: nf_tables: fix set element data type in dumps
* netfilter: nf_tables: fix indentation of struct nft_set_elem comments
* netfilter: nf_tables: fix oops in nft_validate_data_load()
* netfilter: nf_tables: fix oops while listing sets of built-in tables
* netfilter: nf_tables: destroy anonymous sets immediately if binding fails
* netfilter: nf_tables: propagate context to set iter callback
* netfilter: nf_tables: add loop detection

From Pablo Neira Ayuso:
* netfilter: nf_tables: allow to dump all existing sets
* netfilter: nf_tables: fix wrong type for flags variable in newelem

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>