net: iface: Improve thread safety of net_if_ipv*_maddr_* functions

Provide an extra struct net_if * iface parameter to
net_if_ipv*_maddr_join/leave functions, so that the corresponding
interface context, the mcast address belong to, can be locked for the
operation.

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