virtio_balloon: set unspecified page reporting order

virtio_balloon page reporting order is set to MAX_PAGE_ORDER implicitly as
vb->prdev.order is never initialised and is auto-set to zero.

Explicitly mention usage of default page order by making use of
PAGE_REPORTING_ORDER_UNSPECIFIED fallback value.

Link: https://lkml.kernel.org/r/20260303113032.3008371-3-yuvraj.sakshith@oss.qualcomm.com
Signed-off-by: Yuvraj Sakshith <yuvraj.sakshith@oss.qualcomm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Eugenio Pérez <eperezma@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Long Li <longli@microsoft.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>