Revert "powerpc/64s: Remove PROT_SAO support"

This reverts commit 5c9fa16e8a.

Since PROT_SAO can still be useful for certain classes of software,
reintroduce it. Concerns about guest migration for LPARs using SAO
will be addressed next.

Signed-off-by: Shawn Anastasio <shawn@anastas.io>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200821185558.35561-2-shawn@anastas.io

mm/ksm.c

@@ -2453,6 +2453,10 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 		if (vma_is_dax(vma))
 			return 0;
 
+#ifdef VM_SAO
+		if (*vm_flags & VM_SAO)
+			return 0;
+#endif
 #ifdef VM_SPARC_ADI
 		if (*vm_flags & VM_SPARC_ADI)
 			return 0;