vrf: fix packet sniffing for traffic originating from ip tunnels

in commit 04893908
("vrf: add mac header for tunneled packets when sniffer is attached")
an Ethernet header was cooked for traffic originating from tunnel devices.

However, the header is added based on whether the mac_header is unset
and ignores cases where the device doesn't expose a mac header to upper
layers, such as in ip tunnels like ipip and gre.

Traffic originating from such devices still appears garbled when capturing
on the vrf device.

Fix by observing whether the original device exposes a header to upper
layers, similar to the logic done in af_packet.

In addition, skb->mac_len needs to be adjusted after adding the Ethernet
header for the skb_push/pull() surrounding dev_queue_xmit_nit() to work
on these packets.

Fixes: 04893908

 ("vrf: add mac header for tunneled packets when sniffer is attached")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>