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/kernel/ucount.c b/kernel/ucount.c
index 0f9ab3b..66eca94 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -68,6 +68,7 @@
 	}
 static struct ctl_table user_table[] = {
 	UCOUNT_ENTRY("max_user_namespaces"),
+	UCOUNT_ENTRY("max_pid_namespaces"),
 	{ }
 };
 #endif /* CONFIG_SYSCTL */