OSD: FileJournal: call writeq_cond.Signal if necessary in submit_entry

For now, every write op should call writeq_cond.Signal() in FileJournal::submit_entry.
We can call signal if neceesary.

Signed-off-by: Xinze Chi <xmdxcxz@gmail.com>
can just do

 if (writeq.empty())
    writeq_cond.Signal();

above the push_back()... no need for the bool