xen/balloon: Set pages PageOffline() in balloon_add_region()

We are missing a __SetPageOffline(), which is why we can get
!PageOffline() pages onto the balloon list, where
alloc_xenballooned_pages() will complain:

page:ffffea0003e7ffc0 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0xffffe00001000(reserved)
raw: 000ffffe00001000 dead000000000100 dead000000000200 0000000000000000
raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
page dumped because: VM_BUG_ON_PAGE(!PageOffline(page))
------------[ cut here ]------------
kernel BUG at include/linux/page-flags.h:744!
invalid opcode: 0000 [#1] SMP NOPTI

Reported-by: Marek Marczykowski-Górecki <[email protected]>
Tested-by: Marek Marczykowski-Górecki <[email protected]>
Fixes: 77c4adf ("xen/balloon: mark inflated pages PG_offline")
Cc: [email protected] # v5.1+
Cc: Boris Ostrovsky <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Signed-off-by: Boris Ostrovsky <[email protected]>

drivers/xen/balloon.c

@@ -688,6 +688,7 @@ static void __init balloon_add_region(unsigned long start_pfn,
 		/* totalram_pages and totalhigh_pages do not
 		   include the boot-time balloon extension, so
 		   don't subtract from it. */
+		__SetPageOffline(page);
 		__balloon_append(page);
 	}