iscsi class: Fix freeing of skb in get host error path

If get_host_stats failes we are using kfree to free the
skb. We should be using kfree_skb.

This patch was made over Christoph's scsi-queue drivers-for-3.17 branch.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>