mon/PGMap: do not consider changing pg stuck

mon/PGMap: do not consider changing pg stuck

there is chance that we have a PG just created but fails to peered
before a mgr module retrieves the health report from mgr. in that
case, the "last_peered" field is not set, as that pg has not peered. but
normally, the newly created PG will be active+clean in couple seconds
which is way under the default setting of mon_pg_stuck_threshold (60
seconds).

so in this change, if the "last_whatever" is not set, we also use the
"last_changed" as a reference to see if the PG is healthy, and only
consider PG stuck if the last_changed is also too old.

Fixes: https://tracker.ceph.com/issues/45717
Signed-off-by: Kefu Chai <kchai@redhat.com>