libata: don't flush dcache on slab pages

page_mapping() check this via VM_BUG_ON(PageSlab(page)) so we bug here
with the according debuging turned on.

Future TODO: replace this with a flush_dcache_page_for_pio() API
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: stable@kernel.org

drivers/ata/libata-sff.c

@@ -919,7 +919,7 @@ static void ata_pio_sector(struct ata_queued_cmd *qc)
 				       do_write);
 	}
 
-	if (!do_write)
+	if (!do_write && !PageSlab(page))
 		flush_dcache_page(page);
 
 	qc->curbytes += qc->sect_size;