bcachefs: Fix trans->locked assert

in bch2_move_data_btree, we might start with the trans unlocked from a
previous loop iteration - we need a trans_begin() before iter_init().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/move.c

@@ -547,6 +547,7 @@ static int bch2_move_data_btree(struct moving_context *ctxt,
 		ctxt->stats->pos	= BBPOS(btree_id, start);
 	}
 
+	bch2_trans_begin(trans);
 	bch2_trans_iter_init(trans, &iter, btree_id, start,
 			     BTREE_ITER_prefetch|
 			     BTREE_ITER_all_snapshots);