powerpc/mm: handle protnone ptes on fork

We need to mark pages of parent process read only on fork.  Numa fault
pte needs a protnone ptes variant with saved write flag set.  On fork we
need to make sure we remove the saved write bit.  Instead of adding the
protnone check in the caller update ptep_set_wrprotect variants to clear
savedwrite bit.

Without this we see random segfaults in application on fork.

Fixes: c137a275

 ("powerpc/mm/autonuma: switch ppc64 to its own implementation of saved write")
Link: http://lkml.kernel.org/r/1488203787-17849-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Paul Mackerras <paulus@ozlabs.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>