lib/stackdepot: off by one in depot_fetch_stack()

The stack_pools[] array has DEPOT_MAX_POOLS.  The "pools_num" tracks the
number of pools which are initialized.  See depot_init_pool() for more
details.

If pool_index == pools_num_cached, this will read one element beyond what
we want.  If not all the pools are initialized, then the pool will be
NULL, triggering a WARN(), and if they are all initialized it will read
one element beyond the end of the array.

Link: https://lkml.kernel.org/r/361ac881-60b7-471f-91e5-5bf8fe8042b2@moroto.mountain
Fixes: b29d3188 ("lib/stackdepot: store free stack records in a freelist")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/stackdepot.c

@@ -447,7 +447,7 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
 
 	lockdep_assert_not_held(&pool_lock);
 
-	if (pool_index > pools_num_cached) {
+	if (pool_index >= pools_num_cached) {
 		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
 		     pool_index, pools_num_cached, handle);
 		return NULL;