fix: storiface: Make FSOverhead numbers more accurate

[magik6k]

Should bring those numbers closer to reality

[codecov]

Codecov Report

Merging #8481 (0a4b49d) into master (84470e9) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8481      +/-   ##
==========================================
- Coverage   40.74%   40.73%   -0.01%     
==========================================
  Files         686      686              
  Lines       75501    75501              
==========================================
- Hits        30765    30759       -6     
- Misses      39424    39431       +7     
+ Partials     5312     5311       -1     

Impacted Files	Coverage Δ
extern/sector-storage/storiface/filetype.go	91.25% <ø> (ø)
chain/events/state/fastapi.go	75.00% <0.00%> (-25.00%)	⬇️
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
cli/util.go	70.83% <0.00%> (-8.34%)	⬇️
chain/events/events_height.go	68.90% <0.00%> (-2.53%)	⬇️
chain/sub/incoming.go	59.02% <0.00%> (-2.45%)	⬇️
miner/miner.go	56.39% <0.00%> (-0.99%)	⬇️
node/impl/storminer.go	24.10% <0.00%> (-0.32%)	⬇️
paychmgr/simple.go	83.16% <0.00%> (-0.25%)	⬇️
... and 11 more