powerpc/mm: Use _PAGE_READ to indicate Read access

This splits the _PAGE_RW bit into _PAGE_READ and _PAGE_WRITE. It also
removes the dependency on _PAGE_USER for implying read only. Few things
to note here is that, we have read implied with write and execute
permission. Hence we should always find _PAGE_READ set on hash pte
fault.

We still can't switch PROT_NONE to !(_PAGE_RWX). Auto numa depends on
marking a prot none pte _PAGE_WRITE. (For more details look at
b191f9b106 "mm: numa: preserve PTE write permissions across a NUMA
hinting fault")

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/misc/cxl/fault.c

@@ -149,9 +149,9 @@ static void cxl_handle_page_fault(struct cxl_context *ctx,
 	 * update_mmu_cache() will not have loaded the hash since current->trap
 	 * is not a 0x400 or 0x300, so just call hash_page_mm() here.
 	 */
-	access = _PAGE_PRESENT;
+	access = _PAGE_PRESENT | _PAGE_READ;
 	if (dsisr & CXL_PSL_DSISR_An_S)
-		access |= _PAGE_RW;
+		access |= _PAGE_WRITE;
 	if ((!ctx->kernel) || (REGION_ID(dar) == USER_REGION_ID))
 		access |= _PAGE_USER;