ipv6: route: enforce RCU protection in ip6_route_check_nh_onlink()

We need a RCU critical section around rt6_info->from deference, and
proper annotation.

Fixes: 4ed591c8 ("net/ipv6: Allow onlink routes to have a device mismatch if it is the default route")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>