CVE-2026-72297: net: atm: reject out-of-range traffic classes in QoS validation

In the Linux kernel, the following vulnerability has been resolved:

net: atm: reject out-of-range traffic classes in QoS validation

Reject ATM traffic classes above ATMANYCLASS in checktp(). SOATMQOS stores the supplied QoS after checkqos() succeeds, so accepting larger values leaves invalid trafficclass values in vcc->qos.

That bad state later reaches pvcinfo(), which indexes classname[] with vcc->qos.{rx,tp}.trafficclass. Values above ATMANYCLASS cause an out-of-bounds read when /proc/net/atm/pvc is read.

Tighten the existing QoS validation so invalid trafficclass values are rejected at the point where user supplied QoS is accepted.