validate storage class across pools when setting config (#11320)

```
mc admin config set alias/ storage_class standard=EC:3
```

should only succeed if parity ratio is valid for all
server pools, if not we should fail proactively.

This PR also needs to bring other changes now that
we need to cater for variadic drive counts per pool.

Bonus fixes also various bugs reproduced with

- GetObjectWithPartNumber()
- CopyObjectPartWithOffsets()
- CopyObjectWithMetadata()
- PutObjectPart,PutObject with truncated streams

cmd/erasure-server-pool.go

@@ -167,8 +167,12 @@ func (z *erasureServerPools) GetAllLockers() []dsync.NetLocker {
 	return z.serverPools[0].GetAllLockers()
 }
 
-func (z *erasureServerPools) SetDriveCount() int {
-	return z.serverPools[0].SetDriveCount()
+func (z *erasureServerPools) SetDriveCounts() []int {
+	setDriveCounts := make([]int, len(z.serverPools))
+	for i := range z.serverPools {
+		setDriveCounts[i] = z.serverPools[i].SetDriveCount()
+	}
+	return setDriveCounts
 }
 
 type serverPoolsAvailableSpace []poolAvailableSpace
@@ -320,16 +324,19 @@ func (z *erasureServerPools) Shutdown(ctx context.Context) error {
 func (z *erasureServerPools) BackendInfo() (b BackendInfo) {
 	b.Type = BackendErasure
 
-	setDriveCount := z.SetDriveCount()
 	scParity := globalStorageClass.GetParityForSC(storageclass.STANDARD)
 	if scParity <= 0 {
 		scParity = z.serverPools[0].defaultParityCount
 	}
-	b.StandardSCData = setDriveCount - scParity
-	b.StandardSCParity = scParity
-
 	rrSCParity := globalStorageClass.GetParityForSC(storageclass.RRS)
-	b.RRSCData = setDriveCount - rrSCParity
+
+	// Data blocks can vary per pool, but parity is same.
+	for _, setDriveCount := range z.SetDriveCounts() {
+		b.StandardSCData = append(b.StandardSCData, setDriveCount-scParity)
+		b.RRSCData = append(b.RRSCData, setDriveCount-rrSCParity)
+	}
+
+	b.StandardSCParity = scParity
 	b.RRSCParity = rrSCParity
 	return
 }
@@ -1360,7 +1367,7 @@ func (z *erasureServerPools) HealObjects(ctx context.Context, bucket, prefix str
 				// knows that its not our first attempt at 'prefix'
 				err = nil
 
-				if quorumCount == z.SetDriveCount() && opts.ScanMode == madmin.HealNormalScan {
+				if quorumCount == set.setDriveCount && opts.ScanMode == madmin.HealNormalScan {
 					continue
 				}
 
@@ -1482,8 +1489,8 @@ func (z *erasureServerPools) Health(ctx context.Context, opts HealthOptions) Hea
 	reqInfo := (&logger.ReqInfo{}).AppendTags("maintenance", strconv.FormatBool(opts.Maintenance))
 
 	b := z.BackendInfo()
-	writeQuorum := b.StandardSCData
-	if b.StandardSCData == b.StandardSCParity {
+	writeQuorum := b.StandardSCData[0]
+	if writeQuorum == b.StandardSCParity {
 		writeQuorum++
 	}