ipv6: do not leave garbage in rt->fib6_metrics

In case ip_fib_metrics_init() returns an error, we better
rewrite rt->fib6_metrics with &dst_default_metrics so that
we do not crash later in ip_fib_metrics_put()

Fixes: 767a2217 ("net: common metrics init helper for FIB entries")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2976,6 +2976,8 @@ static struct fib6_info *ip6_route_info_create(struct fib6_config *cfg,
 	rt->fib6_metrics = ip_fib_metrics_init(net, cfg->fc_mx, cfg->fc_mx_len);
 	if (IS_ERR(rt->fib6_metrics)) {
 		err = PTR_ERR(rt->fib6_metrics);
+		/* Do not leave garbage there. */
+		rt->fib6_metrics = (struct dst_metrics *)&dst_default_metrics;
 		goto out;
 	}