Sanitize qnx4 fsync handling

* have directory operations use mark_buffer_dirty_inode(),
  so that sync_mapping_buffers() would get those.
* make qnx4_write_inode() honour its last argument.
* get rid of insane copies of very ancient "walk the indirect blocks"
  in qnx4/fsync - they never matched the actual fs layout and, fortunately,
  never'd been called.  Again, all this junk is not needed; ->fsync()
  should just do sync_mapping_buffers + sync_inode (and if we implement
  block allocation for qnx4, we'll need to use mark_buffer_dirty_inode()
  for extent blocks)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>