netfilter: nfnetlink: remove redundant variable nskb

Actually after netlink_skb_clone() is called, the nskb and
skb will point to the same thing, but they are used just like
they are different, sometimes this is confusing, so i think
there is no necessary to keep nskb anymore.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@soleta.eu>