bpf: hashtab: fix 32-bit overflow in memory usage calculation

The intermediate product value_size * num_possible_cpus() is evaluated
in 32-bit arithmetic and only then promoted to 64 bits. On systems with
large value_size and many possible CPUs this can overflow and lead to
an underestimated memory usage.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 304849a ("bpf: hashtab memory usage")
Cc: stable@vger.kernel.org
Suggested-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Alexei Safin <a.safin@rosa.ru>
Acked-by: Yafang Shao <laoar.shao@gmail.com>

Author: Alexei Safin

kernel/bpf/hashtab.c

@@ -2199,7 +2199,7 @@ static long bpf_for_each_hash_elem(struct bpf_map *map, bpf_callback_t callback_
 static u64 htab_map_mem_usage(const struct bpf_map *map)
 {
 	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
-	u32 value_size = round_up(htab->map.value_size, 8);
+	u64 value_size = round_up(htab->map.value_size, 8);
 	bool prealloc = htab_is_prealloc(htab);
 	bool percpu = htab_is_percpu(htab);
 	bool lru = htab_is_lru(htab);