audit_alloc: clear TIF_SYSCALL_AUDIT if !audit_context

If audit_filter_task() nacks the new thread it makes sense
to clear TIF_SYSCALL_AUDIT which can be copied from parent
by dup_task_struct().

A wrong TIF_SYSCALL_AUDIT is not really bad but it triggers
the "slow" audit paths in entry.S to ensure the task can not
miss audit_syscall_*() calls, this is pointless if the task
has no ->audit_context.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Steve Grubb <sgrubb@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

kernel/auditsc.c

@@ -943,8 +943,10 @@ int audit_alloc(struct task_struct *tsk)
 		return 0; /* Return if not auditing. */
 
 	state = audit_filter_task(tsk, &key);
-	if (state == AUDIT_DISABLED)
+	if (state == AUDIT_DISABLED) {
+		clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
 		return 0;
+	}
 
 	if (!(context = audit_alloc_context(state))) {
 		kfree(key);