vdpa_sim: fix cleanup after worker creation failure

In the Linux kernel, the following vulnerability has been resolved: vdpa_sim: fix cleanup after worker creation failure vdpasim_create() leaves vdpasim->worker as an ERR_PTR when kthread_run_worker() fails. The error path then drops the device reference, which releases the partially initialized simulator. vdpasim_free() unconditionally passes the worker pointer to kthread_destroy_worker(), so the ERR_PTR is dereferenced and can trigger a general protection fault. Store the worker error, clear the pointer, and only clean up the worker when it was successfully initialized. Also make the release path tolerate partially initialized objects by guarding virtqueue and IOTLB cleanup, since the same release path can be reached from other initialization failures. I found this bug myself, though the patch was written with AI assistance.