ipv6: fix ipv6_cow_metrics for non DST_HOST case

ipv6_cow_metrics() currently assumes only DST_HOST routes require
dynamic metrics allocation from inetpeer.  The assumption breaks
when ndisc discovered router with RTAX_MTU and RTAX_HOPLIMIT metric.
Refer to ndisc_router_discovery() in ndisc.c and note that dst_metric_set()
is called after the route is created.

This patch creates the metrics array (by calling dst_cow_metrics_generic) in
ipv6_cow_metrics().

Test:
radvd.conf:
interface qemubr0
{
	AdvLinkMTU 1300;
	AdvCurHopLimit 30;

	prefix fd00:face:face:face::/64
	{
		AdvOnLink on;
		AdvAutonomous on;
		AdvRouterAddr off;
	};
};

Before:
[root@qemu1 ~]# ip -6 r show | egrep -v unreachable
fd00:face:face:face::/64 dev eth0  proto kernel  metric 256  expires 27sec
fe80::/64 dev eth0  proto kernel  metric 256
default via fe80::74df:d0ff:fe23:8ef2 dev eth0  proto ra  metric 1024  expires 27sec

After:
[root@qemu1 ~]# ip -6 r show | egrep -v unreachable
fd00:face:face:face::/64 dev eth0  proto kernel  metric 256  expires 27sec mtu 1300
fe80::/64 dev eth0  proto kernel  metric 256  mtu 1300
default via fe80::74df:d0ff:fe23:8ef2 dev eth0  proto ra  metric 1024  expires 27sec mtu 1300 hoplimit 30

Fixes: 8e2ec639

 (ipv6: don't use inetpeer to store metrics for routes.)
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>