simplefs_write(): avoid leaking bh on error
simplefs_write: install h_sync (sync on close) for jbd2 handle

This will make simplefs journaling more robust.

+ head -n100 /proc/fs/jbd2/loop0/info
7 transactions (7 requested), each up to 640 blocks
average:
  0ms waiting for transaction
  10ms request delay
  26ms running transaction
  0ms transaction was being locked
  0ms flushing data (in ordered mode)
  0ms logging transaction
  3459us average transaction commit time
  1 handles per transaction
  1 blocks per transaction
  2 logged blocks per transaction