ppc64 PTE hacks

[Benjamin Herrenschmidt]

> - Code the flush into ptep_test_and_clear_dirty since msync is the only place
>   that uses it.
> - Fix the generic ptep_clear_flush_dirty code to always call flush_tlb_page
> - Create our own version of ptep_clear_flush_dirty
> - Fix hpte_update to not cast out the hpte on DIRTY bit transitions

Or better, do a smarter pte_update that does nothing if the bit wasn't
modified...

Ben.


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