debugobjects: Prepare kmem_cache allocations for batching

Allocate a batch and then push it into the pool. Utilize the
debug_obj::last_node pointer for keeping track of the batch boundary.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241007164914.198647184@linutronix.de