ipv6: Refactor ip6_route_del for cached routes

Move the removal of cached routes to a helper, ip6_del_cached_rt, that
can be invoked per nexthop. Rename the existig ip6_del_cached_rt to
__ip6_del_cached_rt since it is called by ip6_del_cached_rt.

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