netfilter: {ip,ip6,nfnetlink}_queue: misc cleanups

- No need to perform data_len = 0 in the switch command, since data_len
  is initialized to 0 in the beginning of the ipq_build_packet_message()
  method.

- {ip,ip6}_queue: We can reach nlmsg_failure only from one place; skb is
  sure to be NULL when getting there; since skb is NULL, there is no need
  to check this fact and call kfree_skb().

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>