PG: set epoch_created and parent_split_bits for child pg

PG: set epoch_created and parent_split_bits for child pg

if we have a wrong "created", OSD receiving an osd_pg_create message
could fail to project its history and crash. a bad split_bits could
mislead the monitor and osd, and hence cause problems.
* set child.info.history.epoch_created to osdmap.get_epoch() instead of
  using the parent's epoch_created
* set child.info.stats.parent_split_bits to the calculated split_bits
  instead using the parent's split_bits

Fixes: http://tracker.ceph.com/issues/15426
Signed-off-by: Kefu Chai <kchai@redhat.com>