bond: avoid setting "active_slave" option without interface enslaved

Kernel will reject setting "active_slave", if the interface is not enslaved or not
up. We already handle that by setting the option whenever we enslave an interface.
However, we also must not set it initially, otherwise we get an ugly error log message:

    NetworkManager[939]: <debug> [1594709143.7459] platform-linux: sysctl: setting net:/sys/class/net/bond99/bonding/active_slave to eth1 (current value is )
    NetworkManager[939]: <error> [1594709143.7459] platform-linux: sysctl: failed to set bonding/active_slave to eth1: (22) Invalid argument
    NetworkManager[939]: <warn>  [1594709143.7460] device (bond99): failed to set bonding attribute active_slave to eth1
    ...
    kernel: bond99: (slave eth1): Device is not bonding slave
    kernel: bond99: option active_slave: invalid value (eth1)

See-also: https://bugzilla.redhat.com/show_bug.cgi?id=1856640
22 jobs for th/bond-active-slave-warning in 37 minutes and 25 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #3596472
checkpatch

00:01:24

manual #3596479
allowed to fail manual
t_centos:7.5.1804
manual #3596480
allowed to fail manual
t_centos:7.6.1810
manual #3596481
allowed to fail manual
t_centos:7.7.1908
manual #3596482
allowed to fail manual
t_centos:7.8.2003
manual #3596483
allowed to fail manual
t_centos:8.1.1911
manual #3596489
allowed to fail manual
t_debian:10
manual #3596488
allowed to fail manual
t_debian:9
manual #3596491
allowed to fail manual
t_debian:sid
manual #3596490
allowed to fail manual
t_debian:testing
manual #3596473
allowed to fail manual
t_fedora:28
manual #3596474
allowed to fail manual
t_fedora:29
manual #3596475
allowed to fail manual
t_fedora:30
passed #3596476
t_fedora:31

00:37:25

manual #3596477
allowed to fail manual
t_fedora:32
manual #3596478
allowed to fail manual
t_fedora:rawhide
manual #3596484
allowed to fail manual
t_ubuntu:16.04
manual #3596485
allowed to fail manual
t_ubuntu:18.04
manual #3596487
allowed to fail manual
t_ubuntu:devel
manual #3596486
allowed to fail manual
t_ubuntu:rolling
 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1662/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1662/

07:13:21