xen: balloon: update the NR_BALLOON_PAGES state

Update the NR_BALLOON_PAGES counter when pages are added to or removed
from the Xen balloon.

Link: https://lkml.kernel.org/r/20250314213757.244258-5-npache@redhat.com
Signed-off-by: Nico Pache <npache@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Juegren Gross <jgross@suse.com>
Cc: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: PacheNico

drivers/xen/balloon.c

@@ -157,6 +157,8 @@ static void balloon_append(struct page *page)
 		list_add(&page->lru, &ballooned_pages);
 		balloon_stats.balloon_low++;
 	}
+	inc_node_page_state(page, NR_BALLOON_PAGES);
+
 	wake_up(&balloon_wq);
 }
 
@@ -179,6 +181,8 @@ static struct page *balloon_retrieve(bool require_lowmem)
 		balloon_stats.balloon_low--;
 
 	__ClearPageOffline(page);
+	dec_node_page_state(page, NR_BALLOON_PAGES);
+
 	return page;
 }