net: shaper: enforce singleton NETDEV scope with id 0

net: shaper: enforce singleton NETDEV scope with id 0

The NETDEV scope represents a singleton root shaper in the per-device
hierarchy.  All code assumes NETDEV shapers have id 0:
net_shaper_default_parent() hardcodes parent->id = 0 when returning
the NETDEV parent for QUEUE/NODE children, and the UAPI documentation
describes NETDEV scope as "the main shaper" (singular, not plural).

Make sure we reject non-0 IDs.

Fixes: 4b623f9f0f59 ("net-shapers: implement NL get operation")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20260510192904.3987113-10-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>