tools: skip up to expire_pos in journal-tool

Previously worked for journals starting from an
object boundary (i.e. freshly created filesystems)

Fixes: #9977
Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2853 from ceph/wip-9977

tools: skip up to expire_pos in journal-tool

Reviewed-by: Greg Farnum <gfarnum@redhat.com>