build_image: btrfs: minimize allocation

Rebalance the /usr btrfs allocation to the maximum possible, in order to
increase the chance of having the btrfs `Free (statfs, df)` similar to
`Free (estimated)`.

Note that /usr is also a zstd compressed btrfs partition, so the output
of `df` free size and the actual free size after a file write for
example, will be very different, because the data in that file write has
a compression rate only definable after the file sync.

Unfortunately, there is no determinism in the btrfs file system case, because even if
you could in theory pre-compress with zstd the file before, and have an
idea about the size to be used, you still cannot really predict also the metadata
size for that file write.

See: flatcar/Flatcar#1473

Signed-off-by: Adrian Vladu <[email protected]>

build_library/disk_util

@@ -661,6 +661,7 @@ def ReadWriteSubvol(options, partition, disable_rw):
     btrfs_mount = tempfile.mkdtemp()
     Sudo(['mount', '-t', 'btrfs', loop_dev, btrfs_mount])
     try:
+      Sudo(['btrfs', 'balance', 'start', '-dusage=0', '-musage=0', btrfs_mount])
       Sudo(['btrfs', 'property', 'set', '-ts', btrfs_mount, 'ro', 'true' if disable_rw else 'false'])
     finally:
       Sudo(['umount', btrfs_mount])