[NETFILTER]: Fix double free after netlink_unicast() in ctnetlink

It's not necessary to free skb if netlink_unicast() failed.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>