crimson/osd: fold the split-child setup into handle_split_pg_creation

A readability cleanup with no behaviour change.

split_pg()'s loop still did all the per-child setup inline (core
mapping, make_pg, split_colls, split_into, the snapmapper touch) and
then called handle_split_pg_creation() to kick off the child's
PGAdvanceMap. That is a lot of detail for what the loop is really doing.

So let's move that setup into handle_split_pg_creation() and have it
return the child PG. The loop then just asks it to create each child and
collects the result, and the per-child PeeringCtx never has to leave the
function. Children are still created one at a time, each with its own
PeeringCtx.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>