mds/MDBalancer: add an arg to limit depth when dump loads for dirfrags

mds/MDBalancer: add an arg to limit depth when dump loads for dirfrags

Directory hierarchy may be deep for a large filesystem, dump loads would output
a lot and take a long time. So add an arg to limit depth when dump loads of dirfrags.

Fixes: https://tracker.ceph.com/issues/56058
Signed-off-by: haoyixing <haoyixing@kuaishou.com>
(cherry picked from commit a8f138cd3623e42c4f74a18d54ec7dfcc1e89e58)

Conflicts:
  * qa/tasks/cephfs/test_admin.py
    Conflict occurred because test_admin.py on Quincy branch was updated
    by PR #54946.