netfilter: ipset: move function to ip_set_bitmap_ip.c.

One inline function in ip_set_bitmap.h is only called in
ip_set_bitmap_ip.c: move it 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>