netfilter: nft_set_rbtree: check for partial overlaps in anonymous sets

In the Linux kernel, the following vulnerability has been resolved:

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.