When adding unreachable routes and setting the RTNH_F_ONLINK flag, a
device is required to be specified. In Linux kernel 4.16 support for
this flag was added. Until now it was ignored.
If RTNH_F_ONLINK is specified while the device is missing, newer kernels
will respond with No such device.
The result is:
* spam in the log file
* missing routes for both ipv4 and ipv6