x86/mm/pkeys: Add PTE bits for storing protection key

Previous documentation has referred to these 4 bits as "ignored".
That means that software could have made use of them.  But, as
far as I know, the kernel never used them.

They are still ignored when protection keys is not enabled, so
they could theoretically still get used for software purposes.

We also implement "empty" versions so that code that references
to them can be optimized away by the compiler when the config
option is not enabled.

Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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