crimson/osd: Use for loop in PG splitting

crimson/osd: Use for loop in PG splitting

Currently, we use parallel_for_each when splitting a PG into multiple children.
In BlueStore's path, the sequential processing means the last split_collection call
(highest bits child) sets the final parent bits.
In SeaStore with parallel_for_each, separate transactions race, and a lower-bits child can overwrite the correct value.
With incorrect split_bits, the parent PG's list_objects computes a wider hash range than it should,
causing do_delete_work to list and delete objects that actually belong to a child PG.
When that child later tries to access those objects, SeaStore hits an unexpected ENOENT and aborts.

Fixes: https://tracker.ceph.com/issues/74982
Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>