sit: use kfree_skb to replace dev_kfree_skb

In failure case, we should use kfree_skb not
dev_kfree_skb to free skbuff, dev_kfree_skb
is defined as consume_skb.

Trace takes advantage of this point.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>