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>