sub interfaces are not allowed on bond slave

[smradman]

commit c50aff4 introduced a
change that disallows creating subinterfaces (e.g. VLAN) on top of
interfaces that are members of a bond.

"Fix: bond: warn if sub interface is detected on bond slave"

ifupdown2 does not warn but errors out with the message:
bond0 : error: bond0: sub interfaces are not allowed on bond slave: eth1.23

This breaks a setup that is common to users utilizing storage boxes
via iSCSI multipath where a bond is defined on two interfaces and used
for e.g. VM traffic, but a specific VLAN is configured on the bond
members themselves and used as storage network.

Proxmox VE 9 disabled this check, largely reverting the commit.
https://lore.proxmox.com/pve-devel/20250930140948.265119-1-s.hanreich@proxmox.com/T/#u