net: if: Join all unjoined solicited node multicast groups

If an IPv6 address was added to the interface while the
interface was down, its solicited node multicast address
is not joined properly and IPv6 communication will to
other hosts will fail. So make sure to rejoin all the
solicited node multicast groups that were joined already
when the network inteface is going up.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>