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

This provides kernel support for creating bitmap ipsets with comment
support.

As is the case for hashes, this incurs a penalty when flushing or
destroying the entire ipset as the entries must first be walked in order
to free the comment strings. This penalty is of course far less than the
cost of listing an ipset to userspace. Any set created without support
for comments will be flushed/destroyed as before.

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>