netfilter: nft_set_rbtree: check for partial overlaps in anonymous sets

netfilter: nft_set_rbtree: check for partial overlaps in anonymous sets

Userspace provides an optimized representation in case intervals are
adjacent, where the end element is omitted.

The existing partial overlap detection logic skips anonymous set checks
on start elements for this reason.

However, it is possible to add intervals that overlap to this anonymous
where two start elements with the same, eg. A-B, A-C where C < B.

      start     end
	A        B
      start  end
        A     C

Restore the check on overlapping start elements to report an overlap.

Fixes: c9e6978e2725 ("netfilter: nft_set_rbtree: Switch to node list walk for overlap detection")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>