vpn: add route to vpn gw when parent has a default device route

When the parent device has a device default route (i.e. without
gateway) and we establish a VPN on top of it, 'ip route get' for the
VPN gateway returns a device route, which is the same result we get
for an unreachable VPN gateway. However it is necessary to add the
route to the gateway or otherwise it will possibly become unreachable
once the VPN gets activated.

#93
7 jobs for bg/issue93 in 21 minutes and 38 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #98086
allowed to fail
checkpatch

00:09:00

passed #98087
f28_build_autotools

00:21:09

passed #98088
f28_build_meson

00:16:44

passed #98089
f28_rpm_autotools_dbg

00:21:21

passed #98090
f28_rpm_meson_dbg

00:21:07

passed #98091
f_latest_build_autotools

00:21:36

failed #98092
allowed to fail
f_rawhide_build_autotools

00:03:34

 
Name Stage Failure
failed
f_rawhide_build_autotools Test
  CC       src/devices/bluetooth/libnm_device_plugin_bluetooth_la-nm-bluez-device.lo
cc1: all warnings being treated as errors
make[2]: *** [Makefile:14560: src/devices/wwan/libnm_wwan_la-nm-modem-broadband.lo] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make[1]: *** [Makefile:18308: all-recursive] Error 1
make[1]: Leaving directory '/builds/NetworkManager/NetworkManager/build'
make: *** [Makefile:8167: all] Error 2
ERROR: Job failed: exit code 1