net: Add ref counting to net_if_addr struct

Add reference counting to network interface address (for both
IPv4 and IPv6) so that the address is not removed if there are
sockets using it. If the interface address is removed while there
are sockets using it, the connectivity will fail for the said
socket.

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