mds: fix --undump-journal

This hadn't worked for a long time.  This is a fix
for firefly only, as this code was refactored in giant.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2734 from ceph/wip-firefly-undump

mds: fix --undump-journal

Reviewed-by: Sage Weil <sage@redhat.com>