dm table: fix dax_dev NULL dereference in device_synchronous()

If a device doesn't support DAX its 'dax_dev' is NULL.  Fix
device_synchronous() to first check if dax_dev is NULL before
dereferencing it.

Fixes: 2e9ee095 ("dm: enable synchronous dax")
Reported-by: jencce.kernel@gmail.com
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-table.c

@@ -894,7 +894,7 @@ int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
 static int device_synchronous(struct dm_target *ti, struct dm_dev *dev,
 				       sector_t start, sector_t len, void *data)
 {
-	return dax_synchronous(dev->dax_dev);
+	return dev->dax_dev && dax_synchronous(dev->dax_dev);
 }
 
 bool dm_table_supports_dax(struct dm_table *t,