This patch disallows rbtree with single elements, which is causing
problems with the recent timeout support. Before this patch, you
could opt out individual set representations per module, which is
just adding extra complexity.

Fixes: 8d8540c4("netfilter: nft_set_rbtree: add timeout support")
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>