segtree: fix decomposition of unclosed intervals containing address prefixes

segtree: fix decomposition of unclosed intervals containing address prefixes

The code which decomposes unclosed intervals doesn't check for prefixes.  This
leads to incorrect output for sets which contain these.  For example,

  # nft -f - <<END
  table ip t {
    chain c {
      ip saddr 192.0.0.0/2 drop
      ip saddr 10.0.0.0/8 drop
      ip saddr { 192.0.0.0/2, 10.0.0.0/8 } drop
    }
  }
  table ip6 t {
    chain c {
      ip6 saddr ff00::/8 drop
      ip6 saddr fe80::/10 drop
      ip6 saddr { ff00::/8, fe80::/10 } drop
    }
  }
  END
  # nft list table ip6 t
  table ip6 t {
    chain c {
      ip6 saddr ff00::/8 drop
      ip6 saddr fe80::/10 drop
      ip6 saddr { fe80::/10, ff00::-ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff } drop
    }
  }
  # nft list table ip t
  table ip t {
    chain c {
      ip saddr 192.0.0.0/2 drop
      ip saddr 10.0.0.0/8 drop
      ip saddr { 10.0.0.0/8, 192.0.0.0-255.255.255.255 } drop
    }
  }

Instead of treating the final unclosed interval as a special case, reuse the
code which correctly handles closed intervals.

Add a shell test-case.

Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1018156
Fixes: 86b965bdab8d ("segtree: fix decomposition of unclosed intervals")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>