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>
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 0edafe3..f2c5ba5 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -76,7 +76,7 @@
 	struct ucounts *ucounts;
 	int ret, i;
 
-	ret = -EUSERS;
+	ret = -ENOSPC;
 	if (parent_ns->level > 32)
 		goto fail;