qfq: don't leak skb if kzalloc fails

When we need to create a new aggregate to enqueue the skb we call kzalloc.
If that fails we returned ENOBUFS without freeing the skb.

Spotted during code review.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>