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/[e-mail]
Signed-off-by: Nico Pache <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Cc: Alexander Atanasov <[e-mail]>
Cc: Chengming Zhou <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Dexuan Cui <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Juegren Gross <[e-mail]>
Cc: Kanchana P Sridhar <[e-mail]>
Cc: K. Y. Srinivasan <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Nhat Pham <[e-mail]>
Cc: Oleksandr Tyshchenko <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Stefano Stabellini <[e-mail]>
Cc: Wei Liu <[e-mail]>
Cc: Michael Kelley <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/xen/balloon.c

···
 		list_add(&page->lru, &ballooned_pages);
 		balloon_stats.balloon_low++;
 	}
+	inc_node_page_state(page, NR_BALLOON_PAGES);
+
 	wake_up(&balloon_wq);
 }
 
···
 		balloon_stats.balloon_low--;
 
 	__ClearPageOffline(page);
+	dec_node_page_state(page, NR_BALLOON_PAGES);
+
 	return page;
 }