NetworkManager can't control the name of the PPP interface name
created by pppd; so it has to wait for the interface to appear and
then rename it. This happens in nm_device_take_over_link() called by
nm-device-ppp.c:ppp_ifindex_set() when pppd tells NM the ifindex of
the interface that was created.

However, sometimes the initial interface name is already correct, for
example when the connection.interface-name is ppp0 and this is the
first PPP interface created.

When this happens, nm_device_update_from_platform_link() is called on
the NMDevicePPP and it sets the device ifindex. Later, when pppd
notifies NM, nm_device_take_over_link() fails because the ifindex is
already set:

 nm_device_take_over_link: assertion 'priv->ifindex <= 0' failed

Make nm_device_take_over_link() more robust to cope with this
situation.

https://bugzilla.redhat.com/show_bug.cgi?id=1849386
(cherry picked from commit 75bc21c4)