net: ipv6: Rejoin all solicited node mcast addresses

When bringing interface down, all IPv6 multicast addresses are removed
from the interface. However, when the interface was brought back up,
rejoin_ipv6_mcast_groups() would look only for solicited node mcast
addresses already present on the interface. In result, after going back
up, the interface was missing solicited-node mcast addresses for those
unicast addresses, that were already present on the interface when
bringing up.

As net_ipv6_mld_join() does similar checks to skip MLD when not needed,
we can just skip the lookup when rejoining, and use already defined
join_mcast_nodes().

Additionally, check for IPV6 and NO_ND flags on the interface before
attempting to add the address back, those multicast addresses are not
needed if ND is disabled on the interface.

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