remove skbq by immediately free skb after being copied

- couldn't avoid this choice if we also consider shared skb as well. in
  the previous version, if we immediately free only shared skb, there
  could be serious issue when non-shared skb and shared skb are mixed.
  For example, shared skb, which is not inserted into skbq in lbuf,
  could be associated with gc_addr. But, gc handler has no way to find
  until which skb should be freed. It could also be carefully handled by
  comparing skb's address, but now we remove this and will bring it
  again when needed.