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>
Merge pull request #2139 from ceph/wip-journal-header

os/FileJournal: Update the journal header when closing journal

Reviewed-by: Greg Farnum <greg@inktank.com>