netfilter: ipset: move ip_set_comment functions from ip_set.h to ip_set_core.c.

Most of the functions are only called from within ip_set_core.c.

The exception is ip_set_init_comment.  However, this is too complex to
be a good candidate for a static inline function.  Move it to
ip_set_core.c, change its linkage to extern and export it, leaving a
declaration in ip_set.h.

ip_set_comment_free is only used as an extension destructor, so change
its prototype to match and drop cast.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>