message: nydus-image: only write extended sb once
  Right now, when prefetch enabled and xattrs packed,
  extended sb will be revised and rewritten more than
  onece. It is not necessary.
  Only write extended block when all the settings are
  done.
diff: @@ -591,10 +591,6 @@ impl Bootstrap {
  ext_sb.set_blob_table_offset(blob_table_offset);
  ext_sb.set_blob_table_size(blob_table_size as u32);
  - ext_sb
  - .store(&mut bootstrap_writer)
  - .context("failed to store extended SB")?;
  -
  // dump devtslot
  bootstrap_writer
  .seek_offset(EROFS_DEVTABLE_OFFSET as u64)
  @@ -640,22 +636,18 @@ impl Bootstrap {
  .context("failed seek prefetch table offset")?;
  pt.store(&mut bootstrap_writer).unwrap();
  -
  - bootstrap_writer.file.seek(SeekFrom::Start(ext_sb_offset))?;
  - ext_sb
  - .store(&mut bootstrap_writer)
  - .context("failed to revise extended SB")?;
  }
  // EROFS does not have inode table, so we lose the chance to decide if this
  // image has xattr. So we have to rewrite extended super block.
  if ctx.has_xattr {
  ext_sb.set_has_xattr();
  + }
  +
  bootstrap_writer.file.seek(SeekFrom::Start(ext_sb_offset))?;
  ext_sb
  .store(&mut bootstrap_writer)
  - .context("failed to revise extended SB")?;
  - }
  + .context("failed to write extended super block")?;
  // Flush remaining data in BufWriter to file
  bootstrap_writer
  