Explore fiemap block sizes that match archive chunk size

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The high granularity of fiemap metadata (which represent Btrfs, XFS data fragments as small as 4KB) may be slowing down the delta processing phase of send and monitor.

Since the fiemap utility filefrag has an option to format the output by a user-set block size, delta processing might run noticeably quicker with a blocksize set to 64KB or larger to match the archive's chunk size.