device: downgrade warning about IPv6 MTU if IPv6 is disabled

If IPv6 is disabled, changing the IPv6 MTU fails and NM complains with
a warning. Since this error is expected and doesn't do any harm,
downgrade the logging level to DEBUG.

Since IPv6 kernel support can be built as a module, we have to check
the existence of /proc/sys/net/ipv6 every time. Instead of checking it
and then setting the MTU (adding one /proc access for everyone), just try
to set the MTU; in case of failure, determine the reason for the error.

https://bugzilla.redhat.com/show_bug.cgi?id=1840989
22 jobs for bg/ipv6-mtu-warning-rh1840989 in 30 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #3770728
checkpatch

00:01:13

manual #3770735
allowed to fail manual
t_centos:7.5.1804
manual #3770736
allowed to fail manual
t_centos:7.6.1810
manual #3770737
allowed to fail manual
t_centos:7.7.1908
manual #3770738
allowed to fail manual
t_centos:7.8.2003
manual #3770739
allowed to fail manual
t_centos:8.1.1911
manual #3770745
allowed to fail manual
t_debian:10
manual #3770744
allowed to fail manual
t_debian:9
manual #3770747
allowed to fail manual
t_debian:sid
manual #3770746
allowed to fail manual
t_debian:testing
manual #3770729
allowed to fail manual
t_fedora:28
manual #3770730
allowed to fail manual
t_fedora:29
manual #3770731
allowed to fail manual
t_fedora:30
passed #3770732
t_fedora:31

00:30:00

manual #3770733
allowed to fail manual
t_fedora:32
manual #3770734
allowed to fail manual
t_fedora:rawhide
manual #3770740
allowed to fail manual
t_ubuntu:16.04
manual #3770741
allowed to fail manual
t_ubuntu:18.04
manual #3770743
allowed to fail manual
t_ubuntu:devel
manual #3770742
allowed to fail manual
t_ubuntu:rolling
 
  External
passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1677/

passed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1677/

06:19:06