netlabel: netlink_unicast calls kfree_skb on error path by itself

So, no need to kfree_skb here on the error path. In this case we can
simply return.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>