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/utsname.c

@@ -49,7 +49,7 @@ static struct uts_namespace *clone_uts_ns(struct user_namespace *user_ns,
 	struct ucounts *ucounts;
 	int err;
 
-	err = -ENFILE;
+	err = -ENOSPC;
 	ucounts = inc_uts_namespaces(user_ns);
 	if (!ucounts)
 		goto fail;