netfilter: nf_tables: honor set timeout and garbage collection updates

Set timeout and garbage collection interval updates are ignored on
updates. Add transaction to update global set element timeout and
garbage collection interval.

Fixes: 96518518 ("netfilter: add nftables")
Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>