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>

fs/namespace.c

@@ -2754,7 +2754,7 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns)
 
 	ucounts = inc_mnt_namespaces(user_ns);
 	if (!ucounts)
-		return ERR_PTR(-ENFILE);
+		return ERR_PTR(-ENOSPC);
 
 	new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
 	if (!new_ns) {