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>
PG:: filter_snapc: Return immediately if no snapc need to trim
we can return immediately if no snapc need to trim. Do not iterater snapc vector and do extra judgement and ops.

Signed-off-by: Ning Yao <zay11022@gmail.com>
AsyncMessenger: fix the leak of file_events

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessenger: reset newly allocated file_events

* otherwise there is chance that Event could take them as already
  initialized ones even they are filled with random bits from heap.
AsyncMessenger: should retry in case of EINTR

* if EINTR is returned, do_send() should return 0 or `continue`.
* reword the comment a little bit

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessenger: s/sended/sent/

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessender: use IOV_MAX instead of IOV_LEN

Signed-off-by: Kefu Chai <tchaikov@gmail.com>