[PATCH] user namespace: fix copy_user_ns return value

[Serge E. Hallyn]

As found by sukadev@xxxxxxxxxx, current -git, when compiled with
CONFIG_USER_NS=n, can be crashed by doing

#include <sched.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
	unshare(CLONE_NEWNS);
	setuid(1000);
}

as root.

The dummy version of copy_user_ns was returning NULL instead of
a valid namespace.  The following patch fixes the problem.
Please apply.

thanks,
-serge