powerpc/mm: Don't clear _PAGE_COHERENT when _PAGE_SAO is set

The current low level hash code on LPAR configurations clears
_PAGE_COHERENT (M) when either _PAGE_GUARDED (G) or _PAGE_NO_CACHE (I)
is set. This conflicts with _PAGE_SAO which has M, I and W bits sets at
once (normally invalid combo) to indicate the new SAO attribute.

This changes the code to allow that case.
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/lpar.c

@@ -305,7 +305,8 @@ static long pSeries_lpar_hpte_insert(unsigned long hpte_group,
 	flags = 0;
 
 	/* Make pHyp happy */
-	if (rflags & (_PAGE_GUARDED|_PAGE_NO_CACHE))
+	if ((rflags & _PAGE_GUARDED) ||
+	    ((rflags & _PAGE_NO_CACHE) & !(rflags & _PAGE_WRITETHRU)))
 		hpte_r &= ~_PAGE_COHERENT;
 
 	lpar_rc = plpar_pte_enter(flags, hpte_group, hpte_v, hpte_r, &slot);