net: veth: fix packet segmentation in veth_convert_skb_to_xdp_buff

Based on the previous allocated packet, page_offset can be not null
in veth_convert_skb_to_xdp_buff routine.
Take into account page fragment offset during the skb paged area copy
in veth_convert_skb_to_xdp_buff().

Fixes: 2d0de67d ("net: veth: use newly added page pool API for veth with xdp")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Yunsheng Lin <linyunsheng@huawei.com>
Link: https://lore.kernel.org/r/eddfe549e7e626870071930964ac3c38a1dc8068.1701702000.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>