netfilter: nf_tables: consolidate set description

[ Upstream commit bed4a63e ]

Add the following fields to the set description:

- key type
- data type
- object type
- policy
- gc_int: garbage collection interval)
- timeout: element timeout

This prepares for stricter set type checks on updates in a follow up
patch.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: f6594c37 ("netfilter: nf_tables: perform type checking for existing sets")
Signed-off-by: Sasha Levin <sashal@kernel.org>