skb_to_sgvec() casuses sg_pcopy_to_buffer() wrong

[Hayes Wang]

Use skb_to_sgvec() to set scatter list, and sometime we would get a
sg->offset which is more than PAGE_SIZE. Call sg_pcopy_to_buffer()
with this scatter list would get wrong data.

In sg_miter_get_next_page(), you would get wrong miter->__remaining,
when the sg->offset is more than PAGE_SIZE.

static bool sg_miter_get_next_page(struct sg_mapping_iter *miter)
{
        if (!miter->__remaining) {
                struct scatterlist *sg;
                unsigned long pgoffset;

                if (!__sg_page_iter_next(&miter->piter))
                        return false;

                sg = miter->piter.sg;
                pgoffset = miter->piter.sg_pgoffset;

                miter->__offset = pgoffset ? 0 : sg->offset;
                miter->__remaining = sg->offset + sg->length -
                                (pgoffset << PAGE_SHIFT) - miter->__offset;
                miter->__remaining = min_t(unsigned long, miter->__remaining,
                                           PAGE_SIZE - miter->__offset);
        }

        return true;
}

Best Regards,
Hayes