debugobjects: Fix various data races

The counters obj_pool_free, and obj_nr_tofree, and the flag obj_freeing are
read locklessly outside the pool_lock critical sections. If read with plain
accesses, this would result in data races.

This is addressed as follows:

 * reads outside critical sections become READ_ONCE()s (pairing with
   WRITE_ONCE()s added);

 * writes become WRITE_ONCE()s (pairing with READ_ONCE()s added); since
   writes happen inside critical sections, only the write and not the read
   of RMWs needs to be atomic, thus WRITE_ONCE(var, var +/- X) is
   sufficient.

The data races were reported by KCSAN:

  BUG: KCSAN: data-race in __free_object / fill_pool

  write to 0xffffffff8beb04f8 of 4 bytes by interrupt on cpu 1:
   __free_object+0x1ee/0x8e0 lib/debugobjects.c:404
   __debug_check_no_obj_freed+0x199/0x330 lib/debugobjects.c:969
   debug_check_no_obj_freed+0x3c/0x44 lib/debugobjects.c:994
   slab_free_hook mm/slub.c:1422 [inline]

  read to 0xffffffff8beb04f8 of 4 bytes by task 1 on ...