debugobjects: Prepare for batching

Move the debug_obj::object pointer into a union and add a pointer to the
last node in a batch. That allows to implement batch processing efficiently
by utilizing the stack property of hlist:

When the first object of a batch is added to the list, then the batch
pointer is set to the hlist node of the object itself. Any subsequent add
retrieves the pointer to the last node from the first object in the list
and uses that for storing the last node pointer in the newly added object.

Add the pointer to the data structure and ensure that all relevant pool
sizes are strictly batch sized. The actual batching implementation follows
in subsequent changes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/all/20241007164914.139204961@linutronix.de