netfilter: ipset: fix ip_set_byindex function

New function added by "Introduction of new commands and protocol
version 7" is not working, since we return skb2 to user

Signed-off-by: Victorien Molle <victorien.molle@wifirst.fr>
Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>