make-disk-image: Account for the ext4 reserved space
make-disk-image: Account for the ext4 reserved space

Reserved space includes:

 - inodes space in use (2 blocks per)
 - about 5.2% of the space

The 5.2% reserved space was computed empirically when working on a
previous EXT4 image builder. It seems to stabilize around 5% even for
much larger filesystems.

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