s3: export PartSize parameter

By default AWS SDK use part_size of 5 MB. For big files,
it is not ideal case. For Hadoop, it is not uncommon to
use 512 MB.

vfs/vfs.go

@@ -103,6 +103,9 @@ func ValidateS3FsConfig(config *S3FsConfig) error {
 			config.KeyPrefix += "/"
 		}
 	}
+	if config.PartSize != 0 && config.PartSize < 5 {
+		return errors.New("part_size ret cannot be lower than 5MB")
+	}
 	return nil
 }