os/FileJournal: Update the journal header when closing journal

When closing journal, it should check must_write_header and update
journal header if must_write_header alreay set.
It can reduce the nosense journal-replay after restarting osd.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Reviewed-by: Sage Weil <sage@redhat.com>
os/FileJournal: signal aio_cond even if seq is 0

This can happen if we write a journal but no events.

Reported-by: Somnath Roy <somnath.roy@sandisk.com>
Reported-by: Ma, Jianpeng <jianpeng.ma@intel.com>
Signed-off-by: Sage Weil <sage@redhat.com>