I_DIRTY_DATASYNC is a strict superset of I_DIRTY_SYNC semantics, as
in mark dirty to be written out by fdatasync as well.  So dirtying
for both flags makes no sense.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>