Don't allow chmod() on the /proc/<pid>/ files

Don't allow chmod() on the /proc/<pid>/ files

This just turns off chmod() on the /proc/<pid>/ files, since there is no
good reason to allow it, and had we disallowed it originally, the nasty
/proc race exploit wouldn't have been possible.

The other patches already fixed the problem chmod() could cause, so this
is really just some final mop-up..

This particular version is based off a patch by Eugene and Marcel which
had much better naming than my original equivalent one.

Signed-off-by: Eugene Teo <[email protected]>
Signed-off-by: Marcel Holtmann <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>