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