ipv6: unshare inetpeers

We currently cow metrics a bit too soon in IPv6 case : All routes are
tied to a single inetpeer entry.

Change ip6_rt_copy() to get destination address as second argument, so
that we fill rt6i_dst before the dst_copy_metrics() call.

icmp6_dst_alloc() must set rt6i_dst before calling dst_metric_set(), or
else the cow is done while rt6i_dst is still NULL.

If orig route points to readonly metrics, we can share the pointer
instead of performing the memory allocation and copy.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>