virtio-net: enable mrg buf header in tap on linux

virtio-net: enable mrg buf header in tap on linux

Modern linux supports arbitrary header size,
which makes it possible to pass mrg buf header
to tap directly without iovec mangling.
Use this capability when it is there.

This removes the need to deal with it in
vhost-net as we do now.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>