journal: drop O_DSYNC for epoch and config if journaling is enabled

Of course writing epoch log or config rarely happens than writing
objects, O_DSYNC for the write should be dropped when the journaling
is enabled.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>