mds/MDLog: try to commit the open file table when there has diry items

mds/MDLog: try to commit the open file table when there has diry items

When there have dirty items, maybe there has no any new log event,
for exmaple when opening the files it can skip the OPEN logs.

Or maybe the journal segments could be expired but the open file
table dirty items still in the cache.

Signed-off-by: Xiubo Li <xiubli@redhat.com>