sit: allow to configure 6rd tunnels via netlink

This patch add the support of 6RD tunnels management via netlink.
Note that netdev_state_change() is now called when 6RD parameters are updated.

6RD parameters are updated only if there is at least one 6RD attribute.

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