lib/stackdepot: store free stack records in a freelist

Instead of using the global pool_offset variable to find a free slot when
storing a new stack record, mainlain a freelist of free slots within the
allocated stack pools.

A global next_stack variable is used as the head of the freelist, and the
next field in the stack_record struct is reused as freelist link (when the
record is not in the freelist, this field is used as a link in the hash
table).

This is preparatory patch for implementing the eviction of stack records
from the stack depot.

Link: https://lkml.kernel.org/r/b9e4c79955c2121b69301778643b203d3fb09ccc.1700502145.git.andreyknvl@google.com


Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>