mds: enhance scrub to fragment/merge dirfrags
Typically, this can only be induced via a client workload. That can be expensive due to the caps generated. See MDBalancer::maybe_fragment for how to trigger fragment/merge. The code change in the MDS should be trivial. The real work is in writing the tests. Suggest a few unit tests which create a directory hierarchy, flush the MDS journal, modify fragment/merge configs, issue a scrub, and check the fragment/merge happened as expected.