dagstore pieceReader: Always read full in ReadAt

[magik6k]

Probably want this in 1.13.2

[codecov]

Codecov Report

Merging #7737 (84c48de) into master (9d14342) will increase coverage by 0.00%.
The diff coverage is 82.35%.

@@           Coverage Diff           @@
##           master    #7737   +/-   ##
=======================================
  Coverage   39.56%   39.57%           
=======================================
  Files         640      640           
  Lines       68350    68364   +14     
=======================================
+ Hits        27042    27052   +10     
- Misses      36658    36664    +6     
+ Partials     4650     4648    -2     

Impacted Files	Coverage Δ
metrics/metrics.go	100.00% <ø> (ø)
markets/dagstore/piecereader.go	60.81% <82.35%> (+4.14%)	⬆️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
chain/exchange/client.go	52.96% <0.00%> (-1.70%)	⬇️
extern/storage-sealing/fsm.go	55.77% <0.00%> (-1.60%)	⬇️
itests/kit/blockminer.go	93.65% <0.00%> (-1.59%)	⬇️
chain/stmgr/searchwait.go	66.02% <0.00%> (-1.29%)	⬇️
extern/sector-storage/sched.go	84.77% <0.00%> (-0.83%)	⬇️
storage/wdpost_run.go	69.62% <0.00%> (-0.64%)	⬇️
chain/sync_manager.go	66.77% <0.00%> (-0.63%)	⬇️
... and 13 more

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