device: set MTU after attaching bond port

When attaching a bond port, kernel will reset the MTU of the port ([1],
[2]). Configuring a different MTU on the port seems not a sensible
thing for the user to do.

Still, before commit e67ddd82 ('device: commit MTU during stage2')
we would first attach the bond port before setting the MTU. That
changed, and now the MTU set by kernel wins.

Btw, this change in behavior happens because we attach the port in
stage3 (ip-config), which seems an ugly thing to do.

Anyway, fix this by setting the MTU after attaching the ports, but still
in stage3.

It is probably not sensible for the user to configure a different MTU.
Still, if the user requested it by configuration, we should apply it.
Note that NetworkManager has some logic to constrain the MTU based on
the parent/child and controller/port. In many regards, NetworkManager
does not fully understand or enforce the correct MTU and relies on the
user to configure it correctly. After all, if the user misconfigures the
MTU, the setup will have problems anyway (even if kernel or NetworkManager
could not trivially see that it's nonsensical).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/bonding/bond_main.c?h=v5.17#n3603
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/bonding/bond_main.c?h=v5.17#n4372

https://bugzilla.redhat.com/show_bug.cgi?id=2071985

Fixes: e67ddd82 ('device: commit MTU during stage2')
57 jobs for th/bond-port-mtu in 31 minutes and 51 seconds (queued for 10 seconds)