vrf: Check skb for XFRM_TRANSFORMED flag

[ Upstream commit 16b9db1ce34ff00d6c18e82825125cfef0cdfb13 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>