drm/i915: fix task reference leak in i915_debugfs.c

Leak a task reference in i915_ppgtt_info(), add put_task_struct()
to fix it.

Introduced by

commit 1c60fef
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:30 2013 -0800

    drm/i915: Dump all ppgtt

Signed-off-by: Geliang Tang <geliangtang@163.com>
[danvet: Add note provided by Jani about which commit introduced this
issue.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2288,9 +2288,13 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
 
 	list_for_each_entry_reverse(file, &dev->filelist, lhead) {
 		struct drm_i915_file_private *file_priv = file->driver_priv;
+		struct task_struct *task;
 
-		seq_printf(m, "\nproc: %s\n",
-			   get_pid_task(file->pid, PIDTYPE_PID)->comm);
+		task = get_pid_task(file->pid, PIDTYPE_PID);
+		if (!task)
+			return -ESRCH;
+		seq_printf(m, "\nproc: %s\n", task->comm);
+		put_task_struct(task);
 		idr_for_each(&file_priv->context_idr, per_file_ctx,
 			     (void *)(unsigned long)m);
 	}