stackdepot: stop preallocating after the final pool

depot_init_pool() decides whether another pool is needed before it
increments pools_num. When it registers the last allowed pool, pools_num
is still one below stack_max_pools, so the existing comparison clears
new_pool.

With new_pool cleared, stack_depot_save_flags() can allocate another
order-2 pool after a lookup miss. depot_keep_new_pool() retains its pointer
in the global new_pool, so the allocation is not lost. However, pools_num
has already reached stack_max_pools, and depot_init_pool() rejects every
attempt to register the pool. It remains allocated and unusable until
reboot. The non-NULL pointer prevents later saves from allocating more
spare pools.

Account for the pool being registered in the limit check so registering
the final pool installs STACK_DEPOT_POISON and prevents the extra
allocation.

Link: https://lore.kernel.org/20261022083330.3159793-1-matt@readmodwrite.com
Fixes: 31639fd6ce ("stackdepot: use variable size records for non-evictable entries")
Signed-off-by: Caleb Kan <ckan@cloudflare.com>
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Caleb Kan <calebkan1106@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>

lib/stackdepot.c

@@ -323,7 +323,7 @@ static bool depot_init_pool(void **prealloc)
 	 * NULL; do not reset to NULL if we have reached the maximum number of
 	 * pools.
 	 */
-	if (pools_num < stack_max_pools)
+	if (pools_num + 1 < stack_max_pools)
 		WRITE_ONCE(new_pool, NULL);
 	else
 		WRITE_ONCE(new_pool, STACK_DEPOT_POISON);