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

drivers/net/veth.c

@@ -790,7 +790,8 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 
 			skb_add_rx_frag(nskb, i, page, page_offset, size,
 					truesize);
-			if (skb_copy_bits(skb, off, page_address(page),
+			if (skb_copy_bits(skb, off,
+					  page_address(page) + page_offset,
 					  size)) {
 				consume_skb(nskb);
 				goto drop;