os/FileJournal: When dump journal, using correctly seq avoid misjudging joural corrupt.

In func FileJournal::dump, it always using seq=0 as last-seq and it can
misjudge the journal corrupt.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2166 from majianpeng/bug-fix

os/FileJournal: When dump journal, using correctly seq avoid misjudging joural corrupt.

Reviewed-by: Samuel Just <sam.just@inktank.com>