userns: When the per user per user namespace limit is reached return ENOSPC

The current error codes returned when a the per user per user
namespace limit are hit (EINVAL, EUSERS, and ENFILE) are wrong.  I
asked for advice on linux-api and it we made clear that those were
the wrong error code, but a correct effor code was not suggested.

The best general error code I have found for hitting a resource limit
is ENOSPC.  It is not perfect but as it is unambiguous it will serve
until someone comes up with a better error code.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/pid_namespace.c

@@ -98,7 +98,7 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	int i;
 	int err;
 
-	err = -EINVAL;
+	err = -ENOSPC;
 	if (level > MAX_PID_NS_LEVEL)
 		goto out;
 	ucounts = inc_pid_namespaces(user_ns);