vmagent: fix calculation of vm_persistentqueue_free_disk_space_bytes (#10271)

### Describe Your Changes

follow up https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10242,
see discussion in
https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10267#issuecomment-3729577415
for more context

### Checklist

The following checks are **mandatory**:

- [x] My change adheres to [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/victoriametrics/contributing/#pull-request-checklist).
- [x] My change adheres to [VictoriaMetrics development
goals](https://docs.victoriametrics.com/victoriametrics/goals/).

lib/persistentqueue/fastqueue.go

@@ -70,7 +70,10 @@ func MustOpenFastQueue(path, name string, maxInmemoryBlocks int, maxPendingBytes
 		if maxPendingBytes == 0 {
 			return float64(freeSpaceBytes)
 		}
-		return min(float64(maxPendingBytes), float64(freeSpaceBytes))
+		fq.mu.Lock()
+		curPendingBytes := fq.pq.GetPendingBytes()
+		fq.mu.Unlock()
+		return min(float64(maxPendingBytes)-float64(curPendingBytes), float64(freeSpaceBytes))
 	})
 
 	pendingBytes := fq.GetPendingBytes()