mm/vmstat: simplify the array size calculation

We can replace the array_num * sizeof(array[0]) with sizeof(array) to
simplify the code.

Link: https://lkml.kernel.org/r/20210715122911.15700-3-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>