netfilter: ipset: move functions to ip_set_core.c.

Several inline functions in ip_set.h are only called in ip_set_core.c:
move them and remove inline function specifier.

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