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

tools/testing/selftests/net/forwarding/lib.sh

@@ -868,6 +868,15 @@ mac_get()
 	ip -j link show dev $if_name | jq -r '.[]["address"]'
 }
 
+ipv6_lladdr_get()
+{
+	local if_name=$1
+
+	ip -j addr show dev $if_name | \
+		jq -r '.[]["addr_info"][] | select(.scope == "link").local' | \
+		head -1
+}
+
 bridge_ageing_time_get()
 {
 	local bridge=$1