md: Don't try to set an array to 'read-auto' if it is already in that state.

'read-auto' is a variant of 'readonly' which will switch to writable
on the first write attempt.

Calling do_md_stop to set the array readonly when it is already readonly
returns an error.  So make sure not to do that.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -2725,9 +2725,9 @@ array_state_store(mddev_t *mddev, const char *buf, size_t len)
 		break;
 	case read_auto:
 		if (mddev->pers) {
-			if (mddev->ro != 1)
+			if (mddev->ro == 0)
 				err = do_md_stop(mddev, 1, 0);
-			else
+			else if (mddev->ro == 1)
 				err = restart_array(mddev);
 			if (err == 0) {
 				mddev->ro = 2;