accel/amdxdna: Fix use-after-free of mm_struct in job scheduler

In the Linux kernel, the following vulnerability has been resolved: accel/amdxdna: Fix use-after-free of mm_struct in job scheduler amdxdna_cmd_submit() stores current->mm in job->mm without holding any reference. aie2_sched_job_run() later access job->mm from the DRM scheduler worker thread. With only a raw pointer and no structural reference, the mm_struct can be freed before the scheduler runs the job. Fix this by calling mmgrab() to hold a structural mm_count reference for the lifetime of the job, paired with mmdrop() in every cleanup path.