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/fs-v1.go

@@ -187,9 +187,9 @@ func (fs *FSObjects) NewNSLock(bucket string, objects ...string) RWLocker {
 	return fs.nsMutex.NewNSLock(nil, bucket, objects...)
 }
 
-// SetDriveCount no-op
-func (fs *FSObjects) SetDriveCount() int {
-	return 0
+// SetDriveCounts no-op
+func (fs *FSObjects) SetDriveCounts() []int {
+	return nil
 }
 
 // Shutdown - should be called when process shuts down.
@@ -735,9 +735,9 @@ func (fs *FSObjects) GetObjectNInfo(ctx context.Context, bucket, object string,
 		rwPoolUnlocker = func() { fs.rwPool.Close(fsMetaPath) }
 	}
 
-	objReaderFn, off, length, rErr := NewGetObjectReader(rs, objInfo, opts, nsUnlocker, rwPoolUnlocker)
-	if rErr != nil {
-		return nil, rErr
+	objReaderFn, off, length, err := NewGetObjectReader(rs, objInfo, opts, nsUnlocker, rwPoolUnlocker)
+	if err != nil {
+		return nil, err
 	}
 
 	// Read the object, doesn't exist returns an s3 compatible error.
@@ -748,10 +748,11 @@ func (fs *FSObjects) GetObjectNInfo(ctx context.Context, bucket, object string,
 		nsUnlocker()
 		return nil, toObjectErr(err, bucket, object)
 	}
-	reader := io.LimitReader(readCloser, length)
+
 	closeFn := func() {
 		readCloser.Close()
 	}
+	reader := io.LimitReader(readCloser, length)
 
 	// Check if range is valid
 	if off > size || off+length > size {