EXP: Looks likes superblock copies are marks as UNINITiated ...

but they still contain some used blocks, ie the block bitmap we get contains
some bits.

This can be observed by using this command:
dumpe2fs tests/floppy.raw | grep -E "(UNINIT|free blocks|Backup)"

Each backup block is mark as UNINIT but their free blocks count is less than 8192.
I have 7 of these and the sum of their non-free blocks match with the number partclone
is missing to obtain the same used blocks count than the one reported by the file
system.