rtnl: allow to create device with IFLA_LINK_NETNSID set

This patch adds the ability to create a netdevice in a specified netns and
then move it into the final netns. In fact, it allows to have a symetry between
get and set rtnl messages.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>