osd_types: pg_t: allow is_split to handle checks for splits prior to the most recent

In generate_past_intervals(), we call is_split to see if a new interval has
started.  For a PG that has recently been created due to a split, this may be
called on an epoch prior to the split, such that m_seed is > the new_pg_num. 
If that happens, just return false, since clearly we didn't split off
children during that epoch.

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