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.

Fixes: aac2430 ("accel/amdxdna: Add command execution")
Reviewed-by: Max Zhen <max.zhen@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260716151305.1595780-1-lizhi.hou@amd.com

Author: houlz0507

drivers/accel/amdxdna/amdxdna_ctx.c

@@ -577,6 +577,7 @@ void amdxdna_sched_job_cleanup(struct amdxdna_sched_job *job)
 	amdxdna_arg_bos_put(job);
 	amdxdna_gem_put_obj(job->cmd_bo);
 	dma_fence_put(job->fence);
+	mmdrop(job->mm);
 }
 
 int amdxdna_cmd_submit(struct amdxdna_client *client,
@@ -642,6 +643,7 @@ int amdxdna_cmd_submit(struct amdxdna_client *client,
 
 	job->hwctx = hwctx;
 	job->mm = current->mm;
+	mmgrab(job->mm);
 
 	job->fence = amdxdna_fence_create(hwctx);
 	if (!job->fence) {
@@ -676,6 +678,8 @@ int amdxdna_cmd_submit(struct amdxdna_client *client,
 cmd_put:
 	amdxdna_gem_put_obj(job->cmd_bo);
 free_job:
+	if (job->mm)
+		mmdrop(job->mm);
 	kfree(job);
 	return ret;
 }