netfilter: x_tables: enforce unique and ascending entry points

Harmless from kernel point of view, but iptables assumes that this is
true when decoding a ruleset.

iptables walks the dumped blob from kernel, and, for each entry that
creates a new chain it prints out rule/chain information.
Base chains (hook entry points) are thus only shown when they appear
in the rule blob.  One base chain that is referenced multiple times
in hook blob is then only printed once.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>