netfilter: nft_set_rbtree: prefer sync gc to async worker

[ Upstream commit 7d259f02 ]

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>
Stable-dep-of: 8d738c18 ("netfilter: nf_tables: fix set size with rbtree backend")
Signed-off-by: Sasha Levin <sashal@kernel.org>