netfilter: ipset: Support comments in hash-type ipsets.

This provides kernel support for creating ipsets with comment support.

This does incur a penalty to flushing/destroying an ipset since all
entries are walked in order to free the allocated strings, this penalty
is of course less expensive than the operation of listing an ipset to
userspace, so for general-purpose usage the overall impact is expected
to be little to none.

Signed-off-by: Oliver Smith <oliver@8.c.9.b.0.7.4.0.1.0.0.2.ip6.arpa>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>