debugobjects: Fix the compilation attributes of some global variables

1. Both debug_objects_pool_min_level and debug_objects_pool_size are
   read-only after initialization, change attribute '__read_mostly' to
   '__ro_after_init', and remove '__data_racy'.

2. Many global variables are read in the debug_stats_show() function, but
   didn't mask KCSAN's detection. Add '__data_racy' for them.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240904133944.2124-2-thunder.leizhen@huawei.com