mm: swap: remove duplicate nr_swap_pages decrement in get_swap_page_of_type()

After commit 4f78252, nr_swap_pages is decremented in
swap_range_alloc(). Since cluster_alloc_swap_entry() calls
swap_range_alloc() internally, the decrement in get_swap_page_of_type()
causes double-decrementing.

As a representative userspace-visible runtime example of the impact,
/proc/meminfo reports increasingly inaccurate SwapFree values.  The
discrepancy grows with each swap allocation, and during hibernation
when large amounts of memory are written to swap, the reported value
can deviate significantly from actual available swap space, misleading
users and monitoring tools.  

Remove the duplicate decrement.

Link: https://lkml.kernel.org/r/20251102082456.79807-1-youngjun.park@lge.com
Fixes: 4f78252 ("mm: swap: move nr_swap_pages counter decrement from folio_alloc_swap() to swap_range_alloc()")
Signed-off-by: Youngjun Park <youngjun.park@lge.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: <stable@vger.kernel.org> [6.17+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: youngjuni

mm/swapfile.c

@@ -2005,10 +2005,8 @@ swp_entry_t get_swap_page_of_type(int type)
 			local_lock(&percpu_swap_cluster.lock);
 			offset = cluster_alloc_swap_entry(si, 0, 1);
 			local_unlock(&percpu_swap_cluster.lock);
-			if (offset) {
+			if (offset)
 				entry = swp_entry(si->type, offset);
-				atomic_long_dec(&nr_swap_pages);
-			}
 		}
 		put_swap_device(si);
 	}