ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Serge Hallyn]

Quoting Theodore Ts'o (tytso at mit.edu):
> On Wed, Apr 30, 2014 at 12:16:41AM +0000, Serge Hallyn wrote:
> > I forget the details, but there was another case where I wanted to
> > have the userns which 'owns' the whole fs available.  I guess we'd
> > have to check against that instead of using inode_capable.
> 
> Yes, that sounds right.
> 
> And *please* tell me that that under no circumstances can anyone other
> than root at init_user_ns is allowed to use mknod....

That's the case.  We've considered making exceptions for things like
/dev/null, but in practice bind-mounting devices from the host has
worked out just fine.