Method new

Reads the block group descriptors from the disk. ## Arguments `ext2` - Weak pointer to the EXT2 filesystem. This function does not call `upgrade` on the pointer so, it is valid to pass a semi-constructed `Arc ` pointer (e.g through `Arc::new_cyclic`) though invoking other functions on the group descriptors require the pointer to be fully constructed). `superblock` - Reference to the EXT2 supe

(
        ext2: Weak<Ext2>,
        device: &BlockDevice,
        superblock: &disk::SuperBlock,
    )