9p/virtio: fix off-by-one error in sg list bounds check

Because the value of limit is VIRTQUEUE_NUM, if index is equal to
limit, it will cause sg array out of bounds, so correct the judgement
of BUG_ON.

Link: http://lkml.kernel.org/r/5B63D5F6.6080109@huawei.com


Signed-off-by: Yiwen Jiang <jiangyiwen@huawei.com>
Reported-By: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jun Piao <piaojun@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>