ipv4: Don't miss existing cached metrics in new routes.

Always lookup to see if we have an existing inetpeer entry for
a route.  Let FLOWI_FLAG_PRECOW_METRICS merely influence the
"create" argument to rt_bind_peer().

Also, call rt_bind_peer() unconditionally since it is not
possible for rt->peer to be non-NULL at this point.

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