Change Unlock to defer Unlock, to avoid data race (#2953)

internal/transport/flowcontrol.go

@@ -149,6 +149,7 @@ func (f *inFlow) maybeAdjust(n uint32) uint32 {
  n = uint32(math.MaxInt32)
  }
  f.mu.Lock()
+ defer f.mu.Unlock()
  // estSenderQuota is the receiver's view of the maximum number of bytes the sender
  // can send without a window update.
  estSenderQuota := int32(f.limit - (f.pendingData + f.pendingUpdate))
@@ -169,10 +170,8 @@ func (f *inFlow) maybeAdjust(n uint32) uint32 {
  // is padded; We will fallback on the current available window(at least a 1/4th of the limit).
  f.delta = n
  }
- f.mu.Unlock()
  return f.delta
  }
- f.mu.Unlock()
  return 0
 }