rbstor data race

[magik6k]

==================
WARNING: DATA RACE
Write at 0x00c00041e1b0 by goroutine 919:
  runtime.mapassign_fast64()
      /usr/lib/go/src/runtime/map_fast64.go:93 +0x0
  github.com/lotus-web3/ribs/rbstor.(*rbs).withReadableGroup()
      /home/magik6k/github.com/lotus_web3/ribs/rbstor/rbs.go:245 +0x37d
  github.com/lotus-web3/ribs/rbstor.(*rbs).HashSample()
      /home/magik6k/github.com/lotus_web3/ribs/rbstor/rbs.go:437 +0x9b
  github.com/lotus-web3/ribs/rbdeal.(*ribs).doRetrievalCheck()
      /home/magik6k/github.com/lotus_web3/ribs/rbdeal/retr_checker.go:110 +0x45e
  github.com/lotus-web3/ribs/rbdeal.(*ribs).retrievalChecker()
      /home/magik6k/github.com/lotus_web3/ribs/rbdeal/retr_checker.go:81 +0x3b7
  github.com/lotus-web3/ribs/rbdeal.Open.func5()
      /home/magik6k/github.com/lotus_web3/ribs/rbdeal/ribs.go:204 +0x58

Previous read at 0x00c00041e1b0 by goroutine 1208:
  runtime.mapaccess1_fast64()
      /usr/lib/go/src/runtime/map_fast64.go:13 +0x0
  github.com/lotus-web3/ribs/rbstor.(*rbs).withReadableGroup()
      /home/magik6k/github.com/lotus_web3/ribs/rbstor/rbs.go:225 +0x544
  github.com/lotus-web3/ribs/rbstor.(*rbs).ReadCar()
      /home/magik6k/github.com/lotus_web3/ribs/rbstor/rbs.go:429 +0x9e
  github.com/lotus-web3/ribs/rbdeal.(*ribs).maybeEnsureS3Offload.func2()
      /home/magik6k/github.com/lotus_web3/ribs/rbdeal/car_offload_s3.go:114 +0x257