mm: zswap: use ATOMIC_LONG_INIT to initialize zswap_stored_pages

mm: zswap: use ATOMIC_LONG_INIT to initialize zswap_stored_pages

This is currently the only atomic_long_t variable initialized by
ATOMIC_INIT macro found in the kernel by using `grep -r atomic_long_t |
grep ATOMIC_INIT`

This was introduced in 6e1fa555ec77, in which we modified the type of
zswap_stored_pages to atomic_long_t, but didn't change the initialization.

Link: https://lkml.kernel.org/r/20250226153253.19179-1-sunk67188@gmail.com
Fixes: 6e1fa555ec77 ("mm: zswap: modify zswap_stored_pages to be atomic_long_t")
Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Acked-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>