debugobjects: Move pool statistics into global_pool struct

Keep it along with the pool as that's a hot cache line anyway and it makes
the code more comprehensible.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/all/20241007164914.318776207@linutronix.de