GHSA-x96g-542c-gccq: Use-after-free freeing an armed dynamically-allocated k_timer in Zephyr userspace object disposal

Zephyr's dynamic kernel-object disposal path unrefcheck() in kernel/userspace/userspace.c frees an object's storage (kfree(dyn->data)) once its reference count reaches zero, after running a per-object-type cleanup. The cleanup switch handled only KOBJMSGQ and KOBJSTACK; there was no KOBJTIMER case. A dynamically-allocated, initialized, and armed ktimer keeps its embedded struct timeout dnode linked in the global timeout queue (timeoutq), so freeing the timer storage without cancelling the timeout leaves a dangling node in that queue.

When the timer next expires, the timeout machinery walks timeoutq and invokes ztimerexpirationhandler() on the freed node, dereferencing and writing freed (and reusable) kernel heap in kernel/ISR context. This is a deterministic use-after-free that does not depend on SMP: the queued node is simply never unlinked at free time.

The disposal is reachable from an unprivileged user thread under CONFIGUSERSPACE + CONFIGDYNAMICOBJECTS: a thread that holds the last permission on such a timer drops it via the kobjectrelease() syscall (or by exiting, through kthreadpermsallclear()), and can arm the timer itself via the ktimerstart() syscall. The free and the expiration handler run at kernel privilege while the actor is a user thread, so the bug is a sandbox-escape memory-corruption primitive usable for privilege escalation. The fix adds ktimercleanup() (cancel the timeout and wait for any in-flight handler) and calls it for KOBJTIMER before freeing.