ReplicatedPG::trim_object: use old_snaps for rollback

We need to rollback the old value of snaps, not the
new one.

Fixes: #7638
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1397 from ceph/wip-7638

ReplicatedPG::trim_object: use old_snaps for rollback

Reviewed-by: Sage Weil <sage@inktank.com>