selftests: net: fib-onlink-tests: Set high metric for default IPv6 route

Currently, the test breaks if the SUT already has a default route
configured for IPv6. Fix by adding "metric 9999" to the `ip -6 ro add`
command, so that multiple default routes can coexist.

Fixes: 4ed591c ("net/ipv6: Allow onlink routes to have a device mismatch if it is the default route")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: NipaLocal <nipa@local>

Author: rbmarliere

tools/testing/selftests/net/fib-onlink-tests.sh

@@ -207,7 +207,7 @@ setup()
 		ip -netns ${PEER_NS} addr add ${V6ADDRS[p${n}]}/64 dev ${NETIFS[p${n}]} nodad
 	done
 
-	ip -6 ro add default via ${V6ADDRS[p3]/::[0-9]/::64}
+	ip -6 ro add default via ${V6ADDRS[p3]/::[0-9]/::64} metric 9999
 	ip -6 ro add table ${VRF_TABLE} default via ${V6ADDRS[p7]/::[0-9]/::64}
 
 	set +e