vrf: add mac header for tunneled packets when sniffer is attached

Before this patch, a sniffer attached to a VRF used as the receiving
interface of L3 tunneled packets detects them as malformed packets and
it complains about that (i.e.: tcpdump shows bogus packets).

The reason is that a tunneled L3 packet does not carry any L2
information and when the VRF is set as the receiving interface of a
decapsulated L3 packet, no mac header is currently set or valid.
Therefore, the purpose of this patch consists of adding a MAC header to
any packet which is directly received on the VRF interface ONLY IF:

 i) a sniffer is attached on the VRF and ii) the mac header is not set.

In this case, the mac address of the VRF is copied in both the
destination and the source address of the ethernet header. The protocol
type is set either to IPv4 or IPv6, depending on which L3 packet is
received.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>