tools: fix MDS journal import

Previously it only worked on fresh filesystems which
hadn't been trimmed yet, and resulted in an invalid
trimmed_pos when expire_pos wasn't on an object
boundary.

Fixes: #10025

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit fb29e71f9a97c12354045ad2e128156e503be696)
Merge pull request #2880 from ceph/wip-10025-firefly

#10025/firefly -- tools: fix MDS journal import

Reviewed-by: Greg Farnum <greg@inktank.com>