vrf: Check skb for XFRM_TRANSFORMED flag

To avoid a loop with qdiscs and xfrms, check if the skb has already gone
through the qdisc attached to the VRF device and then to the xfrm layer.
If so, no need for a second redirect.

Fixes: 193125db ("net: Introduce VRF device driver")
Reported-by: Trev Larock <trev@larock.ca>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>