[tools] Increment fileset volume in shard splitter

[linasm]

What this PR does / why we need it:
Reduce the likelihood of collisions when splitting directly into a running cluster.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:
NONE

Does this PR require updating code package or user-facing documentation?:
NONE

[codecov]

Codecov Report

Merging #3674 (bfffcc5) into master (bfffcc5) will not change coverage.
The diff coverage is n/a.

❗ Current head bfffcc5 differs from pull request most recent head f99be7d. Consider uploading reports for the commit f99be7d to get more accurate results

@@          Coverage Diff           @@
##           master   #3674   +/-   ##
======================================
  Coverage    56.8%   56.8%           
======================================
  Files         551     551           
  Lines       61894   61894           
======================================
  Hits        35156   35156           
  Misses      23593   23593           
  Partials     3145    3145           

Flag	Coverage Δ
aggregator	61.6% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.7% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.7% <0.0%> (ø)
msg	74.3% <0.0%> (ø)

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