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/notification-summary.go

@@ -31,9 +31,16 @@ func GetTotalCapacity(ctx context.Context) (capacity uint64) {
 }
 
 // GetTotalUsableCapacity gets the total usable capacity in the cluster.
+// This value is not an accurate representation of total usable in a multi-tenant deployment.
 func GetTotalUsableCapacity(ctx context.Context, s StorageInfo) (capacity float64) {
 	raw := GetTotalCapacity(ctx)
-	ratio := float64(s.Backend.StandardSCData) / float64(s.Backend.StandardSCData+s.Backend.StandardSCParity)
+	var approxDataBlocks float64
+	var actualDisks float64
+	for _, scData := range s.Backend.StandardSCData {
+		approxDataBlocks += float64(scData)
+		actualDisks += float64(scData + s.Backend.StandardSCParity)
+	}
+	ratio := approxDataBlocks / actualDisks
 	return float64(raw) * ratio
 }
 
@@ -47,8 +54,15 @@ func GetTotalCapacityFree(ctx context.Context) (capacity uint64) {
 }
 
 // GetTotalUsableCapacityFree gets the total usable capacity free in the cluster.
+// This value is not an accurate representation of total free in a multi-tenant deployment.
 func GetTotalUsableCapacityFree(ctx context.Context, s StorageInfo) (capacity float64) {
 	raw := GetTotalCapacityFree(ctx)
-	ratio := float64(s.Backend.StandardSCData) / float64(s.Backend.StandardSCData+s.Backend.StandardSCParity)
+	var approxDataBlocks float64
+	var actualDisks float64
+	for _, scData := range s.Backend.StandardSCData {
+		approxDataBlocks += float64(scData)
+		actualDisks += float64(scData + s.Backend.StandardSCParity)
+	}
+	ratio := approxDataBlocks / actualDisks
 	return float64(raw) * ratio
 }