tools/cephfs: fuller header in dump/undump

There were two problems here:
 * write_pos was modified through an undump/dump cycle,
   because it was probed during recovery.
 * stream format was being forgotten.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2159 from ceph/wip-undump

tools/cephfs: fuller header in dump/undump

Reviewed-by: Greg Farnum <greg@inktank.com>