pidns: Add a limit on the number of pid namespaces

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 9b676ea..9ee9482 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -26,6 +26,7 @@
 
 enum ucount_type {
 	UCOUNT_USER_NAMESPACES,
+	UCOUNT_PID_NAMESPACES,
 	UCOUNT_COUNTS,
 };