osd_types: pg_t: add get_ancestor() method

Give us the ancestor for when the pool had a past value for pg_num.

Signed-off-by: Sage Weil <sage@inktank.com>
osd_types: pg_t: make check_new_interval() handle past pg_num values

If we are generated the interval history for a pg that was recently split,
we need make check_new_history behave when the pg_num is smaller than the
latest m_seed.  Do this by passing in the ancestor of the current pg that
existed during that interval.

Fixes: #8139
Signed-off-by: Sage Weil <sage@inktank.com>