powerpc/mm/radix: Update pte update sequence for pte clear case

In the kernel we do follow the below sequence in different code paths.
pte = ptep_get_clear(ptep)
....
set_pte_at(ptep, pte)

We do that for mremap, autonuma protection update and softdirty clearing. This
implies our optimization to skip a tlb flush when clearing a pte update is
not valid, because for DD1 system that followup set_pte_at will be done witout
doing the required tlbflush. Fix that by always doing the dd1 style pte update
irrespective of new_pte value. In a later patch we will optimize the application
exit case.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/radix.h

@@ -144,16 +144,10 @@ static inline unsigned long radix__pte_update(struct mm_struct *mm,
 		 * new value of pte
 		 */
 		new_pte = (old_pte | set) & ~clr;
-		/*
-		 * If we are trying to clear the pte, we can skip
-		 * the below sequence and batch the tlb flush. The
-		 * tlb flush batching is done by mmu gather code
-		 */
-		if (new_pte) {
-			asm volatile("ptesync" : : : "memory");
-			radix__flush_tlb_pte_p9_dd1(old_pte, mm, addr);
+		asm volatile("ptesync" : : : "memory");
+		radix__flush_tlb_pte_p9_dd1(old_pte, mm, addr);
+		if (new_pte)
 			__radix_pte_update(ptep, 0, new_pte);
-		}
 	} else
 		old_pte = __radix_pte_update(ptep, clr, set);
 	asm volatile("ptesync" : : : "memory");