drm: Track clients by tgid and not tid

Thread group id (aka pid from userspace point of view) is a more
interesting thing to show as an owner of a DRM fd, so track and show that
instead of the thread id.

In the next patch we will make the owner updated post file descriptor
handover, which will also be tgid based to avoid ping-pong when multiple
threads access the fd.

Signed-off-by: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Zack Rusin <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Christian König <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c

···
 		 * Therefore, we need to protect this ->comm access using RCU.
 		 */
 		rcu_read_lock();
-		task = pid_task(file->pid, PIDTYPE_PID);
+		task = pid_task(file->pid, PIDTYPE_TGID);
 		seq_printf(m, "pid %8d command %s:\n", pid_nr(file->pid),
 			   task ? task->comm : "<unknown>");
 		rcu_read_unlock();

drivers/gpu/drm/drm_debugfs.c

···
 	seq_printf(m,
 		   "%20s %5s %3s master a %5s %10s\n",
 		   "command",
-		   "pid",
+		   "tgid",
 		   "dev",
 		   "uid",
 		   "magic");
···
 		bool is_current_master = drm_is_current_master(priv);
 
 		rcu_read_lock(); /* locks pid_task()->comm */
-		task = pid_task(priv->pid, PIDTYPE_PID);
+		task = pid_task(priv->pid, PIDTYPE_TGID);
 		uid = task ? __task_cred(task)->euid : GLOBAL_ROOT_UID;
 		seq_printf(m, "%20s %5d %3d   %c    %c %5d %10u\n",
 			   task ? task->comm : "<unknown>",

drivers/gpu/drm/drm_file.c

···
 	if (!file)
 		return ERR_PTR(-ENOMEM);
 
-	file->pid = get_pid(task_pid(current));
+	file->pid = get_pid(task_tgid(current));
 	file->minor = minor;
 
 	/* for compatibility root is always authenticated */

drivers/gpu/drm/vmwgfx/vmwgfx_gem.c

···
 		 * Therefore, we need to protect this ->comm access using RCU.
 		 */
 		rcu_read_lock();
-		task = pid_task(file->pid, PIDTYPE_PID);
+		task = pid_task(file->pid, PIDTYPE_TGID);
 		seq_printf(m, "pid %8d command %s:\n", pid_nr(file->pid),
 			   task ? task->comm : "<unknown>");
 		rcu_read_unlock();