net: failover: use IFF_NO_ADDRCONF flag to prevent ipv6 addrconf

Similar to Bonding and Team, to prevent ipv6 addrconf with
IFF_NO_ADDRCONF in slave_dev->priv_flags for slave ports
is also needed in net failover.

Note that dev_open(slave_dev) is called in .slave_register,
which is called after the IFF_NO_ADDRCONF flag is set in
failover_slave_register().

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>