netfilter: nft_set_rbtree: prefer sync gc to async worker

netfilter: nft_set_rbtree: prefer sync gc to async worker

There is no need for asynchronous garbage collection, rbtree inserts
can only happen from the netlink control plane.

We already perform on-demand gc on insertion, in the area of the
tree where the insertion takes place, but we don't do a full tree
walk there for performance reasons.

Do a full gc walk at the end of the transaction instead and
remove the async worker.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>