tunnel: T5879: properly verify source-interface used for tunnels
A tunnel interface can not properly be sourced from a pppoe0 interface when
such interface is not (yet) connected to the BRAS. It might work on a running
system, but subsequent reboots will fail as the source-interface most likely
does not yet exist.
(cherry picked from commit 66ce19058b7b8597536ddf63bbca027add2ca8a1)