lib/stackdepot: reorder and annotate global variables

Group stack depot global variables by their purpose:

1. Hash table-related variables,
2. Slab-related variables,

and add comments.

Also clean up comments for hash table-related constants.

Link: https://lkml.kernel.org/r/5606a6c70659065a25bee59cd10e57fc60bb4110.1676063693.git.andreyknvl@google.com


Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>