vsock/virtio: use skb_frag_*() helpers

Minor fix for virtio: code wanting to access the fields inside an skb
frag should use the skb_frag_*() helpers, instead of accessing the
fields directly. This allows for extensions where the underlying
memory is not a page.

Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://lore.kernel.org/r/20240102205905.793738-1-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>