vxlan: correctly set vxlan->net when creating the device in a netns

Commit a985343b ("vxlan: refactor verification and application of
configuration") modified vxlan device creation, and replaced the
assignment of vxlan->net to src_net with dev_net(netdev) in ->setup().

But dev_net(netdev) is not the same as src_net. At the time ->setup()
is called, dev_net hasn't been set yet, so we end up creating the
socket for the vxlan device in init_net.

Fix this by bringing back the assignment of vxlan->net during device
creation.

Fixes: a985343b ("vxlan: refactor verification and application of configuration")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David S. Miller <davem@davemloft.net>