net: Store ipv4/ipv6 COW'd metrics in inetpeer cache.

Please note that the IPSEC dst entry metrics keep using
the generic metrics COW'ing mechanism using kmalloc/kfree.

This gives the IPSEC routes an opportunity to use metrics
which are unique to their encapsulated paths.

Signed-off-by: David S. Miller <davem@davemloft.net>