osd: call on_new_interval on newly split child PG

osd: call on_new_interval on newly split child PG

We must call on_new_interval() on any interval change *and* on the
creation of the PG.  Currently we call it from PG::init() and
PG::start_peering_interval().  However, PG::split_into() did not
do so for the child PG, which meant that the new child feature
bits were not properly initialized and the bitwise/nibblewise
debug bit was not correctly set.  That, in turn, could lead to
various misbehaviors, the most obvious of which is scrub errors
due to the sort order mismatch.

Fixes: #13962
Signed-off-by: Sage Weil <sage@redhat.com>