pkeys: Reserve PKEY_DISABLE_READ

[Dave Hansen]

On 11/27/18 3:57 AM, Florian Weimer wrote:
> I would have expected something that translates PKEY_DISABLE_WRITE |
> PKEY_DISABLE_READ into PKEY_DISABLE_ACCESS, and also accepts
> PKEY_DISABLE_ACCESS | PKEY_DISABLE_READ, for consistency with POWER.
> 
> (My understanding is that PKEY_DISABLE_ACCESS does not disable all
> access, but produces execute-only memory.)

Correct, it disables all data access, but not execution.