netfilter: nft_hash: add support for timeouts

Add support for element timeouts to nft_hash. The lookup and walking
functions are changed to ignore timed out elements, a periodic garbage
collection task cleans out expired entries.

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