device: fix reapply of MTU

When we set the MTU on the link we remember its previous source
(ip-config, parent-device or connection profile) and don't change it
again afterwards to avoid interfering with user's manual changes. The
only exceptions when we change it again are (1) if the parent device
MTU changes and (2) if the new MTU has higher priority than the one
previously set.

To allow a live reapply of the MTU property we also need to clear the
saved source, or the checks described above will prevent setting the
new value.

Fixes: 2f891723 ('device: rework mtu priority handling')

https://bugzilla.redhat.com/show_bug.cgi?id=1702657
12 jobs for bg/mtu-reapply-rh1702657 in 26 minutes and 51 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #263851
allowed to fail
checkpatch

00:05:25

passed #263858
t_debian:9

00:13:25

passed #263860
t_debian:sid

00:13:26

manual #263859
allowed to fail manual
t_debian:testing
passed #263852
t_fedora:28

00:20:45

passed #263853
t_fedora:29

00:26:50

passed #263854
t_fedora:30

00:19:21

manual #263855
allowed to fail manual
t_fedora:rawhide
passed #263856
t_ubuntu:16.04

00:13:28

passed #263857
t_ubuntu:18.04

00:13:37

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

06:05:15