FileJournal: Don't cache journal data if journal w/o directio mode

Only read journal data in osd starting.So no need to keep journal
data in kernel cache/buffer.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #3256 from majianpeng/fix4

FileJournal: Don't cache journal data if journal w/o directio mode

Reviewed-by: Sage Weil <sage@redhat.com>