proc/meminfo: expose per-node balloon pages in node meminfo

proc/meminfo: expose per-node balloon pages in node meminfo

Commit 835de37603ef ("meminfo: add a per node counter for balloon
drivers") added NR_BALLOON_PAGES and exposed it in /proc/meminfo.
However, the per-node view at /sys/devices/system/node/nodeX/meminfo was
not updated, even though the counter is already tracked per-node.

Add it to node_read_meminfo() so users can see balloon usage per NUMA node
without having to parse the raw vmstat file.

Link: https://lore.kernel.org/20260509005631.17183-1-hao.ge@linux.dev
Signed-off-by: Hao Ge <hao.ge@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>