FIX: NO SUCH DEVICE when adding routes

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

kernel_netlink.c

@@ -1025,12 +1025,13 @@ kernel_route(int operation, int table,
         rtm->rtm_src_len = src_plen;
     rtm->rtm_table = table;
     rtm->rtm_scope = RT_SCOPE_UNIVERSE;
-    if(metric < KERNEL_INFINITY)
+    if(metric < KERNEL_INFINITY) {
         rtm->rtm_type = RTN_UNICAST;
-    else
+        rtm->rtm_flags |= RTNH_F_ONLINK;
+    } else
         rtm->rtm_type = RTN_UNREACHABLE;
+
     rtm->rtm_protocol = RTPROT_BABEL;
-    rtm->rtm_flags |= RTNH_F_ONLINK;
 
     rta = RTM_RTA(rtm);