[metalos] lower free_mb of some test disks

Summary:
This is probably not necessary after D52542166 does better with resizing the
disk, let's see

Test Plan: waitforsandcastle

Reviewed By: wujj123456

Differential Revision: D52546690

fbshipit-source-id: ebbfa41cada1704e81c8a814407ef7d19d879dd5

antlir/antlir2/antlir2_packager/btrfs_packager/src/main.rs

@@ -180,7 +180,7 @@ pub fn create_empty_file(output: &Path, size: ByteSize) -> Result<()> {
 }
 
 /// Loose estimate of the amount of space that these subvols will take - the
-/// sendstrea size is only correlated to the actual required subvol size in the
+/// sendstream size is only correlated to the actual required subvol size in the
 /// loopback for a few reasons:
 ///  * sendstream has different metadata representations
 ///  * compression levels are almost definitely different