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>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 2081055..e65677d 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -5187,7 +5187,7 @@
 
 		/* Only allow single threaded processes to change context */
 		error = -EPERM;
-		if (!is_single_threaded(p)) {
+		if (!current_is_single_threaded()) {
 			error = security_bounded_transition(tsec->sid, sid);
 			if (error)
 				goto abort_change;