disk_cache: use block size rather than file size

The majority of cache files are less than 1kb this resulted in us
greatly miscalculating the amount of disk space used by the cache.

Using the number of blocks allocated to the file is more
conservative and less likely to cause issues.

This change will result in cache sizes being miscalculated further
until old items added with the previous calculation have all been
removed. However I don't see anyway around that, the previous
patch should help limit that problem.

Cc: "17.1" <[email protected]>
Reviewed-and-Tested-by: Michel Dänzer <[email protected]>

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