kernel: rename is_single_threaded(task) to current_is_single_threaded(void)

- is_single_threaded(task) is not safe unless task == current,
  we can't use task->signal or task->mm.

- it doesn't make sense unless task == current, the task can
  fork right after the check.

Rename it to current_is_single_threaded() and kill the argument.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/keys/process_keys.c

@@ -702,7 +702,7 @@ long join_session_keyring(const char *name)
 	/* only permit this if there's a single thread in the thread group -
 	 * this avoids us having to adjust the creds on all threads and risking
 	 * ENOMEM */
-	if (!is_single_threaded(current))
+	if (!current_is_single_threaded())
 		return -EMLINK;
 
 	new = prepare_creds();