ipv6: Move setting default metric for routes

ip6_route_info_create is a low level function for ensuring fc_metric is
set. Move the check and default setting to the 2 locations that do not
already set fc_metric before calling ip6_route_info_create. This is
required for the next patch which moves addrconf allocations to
ip6_route_info_create and want the metric for host routes to be 0.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>