selftests: forwarding: add helper for retrieving IPv6 link-local address of interface

Pinging an IPv6 link-local multicast address selects the link-local
unicast address of the interface as source, and we'd like to monitor for
that in tcpdump.

Add a helper to the forwarding library which retrieves the link-local
IPv6 address of an interface, to make that task easier.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>