Restore disable_ipv6 sysctl after removing a device from bridge or bond

Restore disable_ipv6 sysctl after removing a device from bridge or bond

If a device is added to a bond or bridge, the disable_ipv6 sysctl is
set, so that no link local addresses are present on the slave devices.

However, the disable_ipv6 sysctl is not restored after removing the
device from the bridge or bond. Therefore, no IPv6 link local addresses
are added and autoconfiguration does not work.

Set the disable_ipv6 to '0' on device removal from bridge or bond.

Signed-off-by: Fabian Bläse <[email protected]>
Link: https://github.com/openwrt/netifd/pull/40
Signed-off-by: Robert Marko <[email protected]>