Implement support for transferring XDP meta data into skb for
virtio_net driver; before calling into the program, xdp.data_meta points
to xdp.data, where on program return with pass verdict, we call
into skb_metadata_set().
Tested with the script at
https://github.com/higebu/virtio_net-xdp-metadata-test.
Signed-off-by: Yuya Kusakabe <yuya.kusakabe@xxxxxxxxx>