FileJournal: stop using sync_file_range

FileJournal: stop using sync_file_range

Using sync_file_range means that neither any required metadata gets commited,
nor the disk cache gets flushed.  Stop using it for the journal, and add
a comment on why a fsync_range system call would be helpful here.

Btw, why does the code use O_SYNC (and not even O_DSYNC!) if using direct
I/O, but fdatasync/fsync for buffered I/O?  Avoiding cache flushes and
metadata updates for every writes is just as important for direct I/O
as it is for buffered I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>