debugobjects: Use static key for boot pool selection

Get rid of the conditional in the hot path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/all/20241007164914.077247071@linutronix.de