drm/amdkfd: hold event_mutex while checkpointing CRIU events

Summary

In the Linux kernel, the following vulnerability has been resolved: drm/amdkfd: hold event_mutex while checkpointing CRIU events kfd_criu_checkpoint_events() counts the entries in p->event_idr via kfd_get_num_events(), allocates an array sized to that count, and then walks the same IDR to fill it. Neither the count nor the walk holds p->event_mutex. The CRIU checkpoint caller holds only p->mutex. Event create and destroy (kfd_event_create()/kfd_event_destroy()) take p->event_mutex and do not take p->mutex, so a second thread in the same process can insert or remove events between the count and the walk. If an event is inserted, the walk iterates more entries than were counted and writes past the end of the ev_privs allocation; if an event is removed, the walk dereferences an entry that is being freed. Hold p->event_mutex across the count and the walk so both observe a consistent view of p->event_idr. The lock is released before copy_to_user(), which only touches the local buffer. The caller already holds p->mutex and the create/destroy paths never take p->mutex, so the p->mutex -> p->event_mutex order is not inverted and no deadlock is introduced. (cherry picked from commit ff57e223ab105795b05d3ef3f3c35a5a441bcbaa)