dm thin metadata: remove in-core 'read_only' flag

Leverage the block manager's read_only flag instead of duplicating it;
access with new dm_bm_is_read_only() method.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/persistent-data/dm-block-manager.h b/drivers/md/persistent-data/dm-block-manager.h
index 1b95dfc..84330f5 100644
--- a/drivers/md/persistent-data/dm-block-manager.h
+++ b/drivers/md/persistent-data/dm-block-manager.h
@@ -123,6 +123,7 @@
  * Additionally you should not use dm_bm_unlock_move, however no error will
  * be returned if you do.
  */
+bool dm_bm_is_read_only(struct dm_block_manager *bm);
 void dm_bm_set_read_only(struct dm_block_manager *bm);
 void dm_bm_set_read_write(struct dm_block_manager *bm);