virtio-net: disable the hole mechanism for xdp

XDP core assumes that the frame_size of xdp_buff and the length of
the frag are PAGE_SIZE. The hole may cause the processing of xdp to
fail, so we disable the hole mechanism when xdp is set.
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -1426,8 +1426,11 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
 		/* To avoid internal fragmentation, if there is very likely not
 		 * enough space for another buffer, add the remaining space to
 		 * the current buffer.
+		 * XDP core assumes that frame_size of xdp_buff and the length
+		 * of the frag are PAGE_SIZE, so we disable the hole mechanism.
 		 */
-		len += hole;
+		if (!headroom)
+			len += hole;
 		alloc_frag->offset += hole;
 	}