fcoe: Use kfree_skb() instead of kfree()

Use kfree_skb() instead of kfree() to free sk_buff.

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>