net: ipv6: Fix missing DAD for link-local address

In case a network interface is brought down and back up, DAD was not
performed for link-local unicast address.

This happens because the logic in the network interface code assumed
that DAD for link-local address is triggered when the address is added,
and it's explicitly omited when looping over IPv6 address. This wasn't
the case however when interface was brought back up, as the address was
already present on the interface, hence DAD skipped.

In Linux, the link-local address is removed from the interface when the
interface is brought down. Such approach solves the issue described,
hence implement it in a similar way in Zephyr.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>