mgr/MMgrReport: default-initialize PerfCounterType members

readable.sh can fail decoding pre-v3 corpus objects that predate the
unit field, e.g.:

  **** reencode of .../objects/PerfCounterType/... resulted in a different dump ****
  7c7
  <     "unit": 36
  ---
  >     "unit": 101

For struct_v < 3 unit is never decoded and had no in-class default, so
it read indeterminate memory and differed between the two ceph-dencoder
processes readable.sh compares. Default it to UNIT_NONE.

Also default-initialize type to PERFCOUNTER_NONE for consistency.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>