netfilter: ipset: Check for comment netlink attribute length

Ensure userspace supplies string not longer than
IPSET_MAX_COMMENT_SIZE.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>