Size reported by fileAllocatedSizeKey incorrect on linux

[patrick-zippenfenig]

Hi,

The allocated file size using fileAllocatedSizeKey is wrongly calculated for linux.

Current implementation:

let stat = try urlStat()
result[key] = Int(stat.st_blocks) * Int(stat.st_blksize)

Proposed implementation:

let stat = try urlStat()
result[key] = Int(stat.st_blocks) * 512

st_blocks appears to be always defined as 512 on Linux. st_blksize is the prefered IO size, but unrelated to the underlaying storage size. As the preferred IO size may be reported as 4096, this results in 8 times larger file size. This is exactly what I am seeing right now, while using fileAllocatedSizeKey.

I am unsure if other platforms may use st_blksize to report a different storage block size.

Let me know if I should submit a PR