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>
(cherry picked from commit 65c33503c83ff8d88781c5c3ae81d88d84c8b3e4)

Conflicts:
	src/tools/cephfs/JournalScanner.cc
Merge pull request #2887 from ceph/wip-9977-backport

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