file-syncfs: Avoid deadlock

This is a four-player problem (here for diskfs):

- One runs sync, which calls diskfs_S_file_syncfs, which triggers
  fsys_syncfs on all active translators, while keeping the
  translator_ihash_lock lock.

- One of the active translators is hung, for some reason

- Another ext2fs thread is trying to call fshelp_set_active_translator from
  dir_lookup. It is stuck on trying to acquire translator_ihash_lock, and it
  holds the np.

- The ext2fs thread running diskfs_sync_everything tries to lock that
  np, while holding the nodecache_lock.

In the end everything is locked. While diskfs_S_file_syncfs can as well just
atomically get the list of active translators, and then call fsys_syncfs
without keeping translator_ihash_lock held.

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