vxlan: setup the right link netns in newlink hdlr

Rename the netns to src_net to avoid confusion with the netns where the
interface stands. The user may specify IFLA_NET_NS_[PID|FD] to create
a x-netns netndevice: IFLA_NET_NS_[PID|FD] points to the netns where the
netdevice stands and src_net to the link netns.

Note that before commit f01ec1c0 ("vxlan: add x-netns support"), it was
possible to create a x-netns vxlan netdevice, but the netdevice was not
operational.

Fixes: f01ec1c0 ("vxlan: add x-netns support")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>