utils: migrate to new get_vrf_tableid() helper

Commit 452068ce7 ("interfaces: T6592: moving an interface between VRF instances
failed") introduced a new helper to retrieve the VRF table ID from the Kernel.
This commit migrates the old code path where the individual fields got queried
to the new helper vyos.utils.network.get_vrf_tableid().

(cherry picked from commit 36f3791e0c15267483d59a3bb74465811d08df88)