vxlan: add support for underlay in non-default VRF

Creating a VXLAN device with is underlay in the non-default VRF makes
egress route lookup fail or incorrect since it will resolve in the
default VRF, and ingress fail because the socket listens in the default
VRF.

This patch binds the underlying UDP tunnel socket to the l3mdev of the
lower device of the VXLAN device. This will listen in the proper VRF and
output traffic from said l3mdev, matching l3mdev routing rules and
looking up the correct routing table.

When the VXLAN device does not have a lower device, or the lower device
is in the default VRF, the socket will not be bound to any interface,
keeping the previous behaviour.

The underlay l3mdev is deduced from the VXLAN lower device
(IFLA_VXLAN_LINK).

+----------+                         +---------+
|          |                         |         |
| vrf-blue |                         | vrf-red |
|          |                         |         |
+----+-----+                         +----+----+
     |                                    |
     |                                    |
+----+-----+                         +----+----+
|          |                         |         |
| br-blue  |                         | br-red  |
|          |                         |         |
+----+-----+                         +---+-+---+
     |                                   | |
     |                             +-----+ +-----+
     |                             |             |
+----+-----+                +------+----+   +----+----+
|          |  lower device  |           |   |         |
|   eth0   | <- - - - - - - | vxlan-red |   | tap-red | (... more taps)
|          |                |           |   |         |
+----------+                +-----------+   +---------+

Signed-off-by: Alexis Bauvin <abauvin@scaleway.com>
Reviewed-by: Amine Kherbouche <akherbouche@scaleway.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Tested-by: Amine Kherbouche <akherbouche@scaleway.com>
Signed-off-by: David S. Miller <davem@davemloft.net>