ipv6: don't use inetpeer to store metrics for routes.

Current IPv6 implementation uses inetpeer to store metrics for
routes. The problem of inetpeer is that it doesn't take subnet
prefix length in to consideration. If two routes have the same
address but different prefix length, they share same inetpeer.
So changing metrics of one route also affects the other. The
fix is to allocate separate metrics storage for each route.

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>