mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

N_NORMAL_MEMORY is initialized from zone population at boot, but memory
hotplug currently only updates N_MEMORY.  As a result, a node that gains
normal memory via hotplug can remain invisible to users iterating over
N_NORMAL_MEMORY, while a node that loses its last normal memory can stay
incorrectly marked as such.

The most visible effect is that
/sys/devices/system/node/has_normal_memory does not report a node even
after that node has gained normal memory via hotplug.

Also, list_lru-based shrinkers can undercount objects on such a node
and may skip reclaim on that node entirely, which can lead to a higher
memory footprint than expected.

Restore N_NORMAL_MEMORY maintenance directly in online_pages() and
offline_pages().  Set the bit when a node that currently lacks normal
memory onlines pages into a zone <= ZONE_NORMAL, and clear it when
offlining removes the last present pages from zones <= ZONE_NORMAL.

This restores the intended semantics without bringing back the old
status_change_nid_normal notifier plumbing which was removed in
8d2882a8edb8.

Current users that benefit include list_lru, zswap, nfsd filecache,
hugetlb_cgroup, and has_normal_memory sysfs reporting.

Link: https://lkml.kernel.org/r/20260330035941.518186-1-hao.li@linux.dev
Fixes: 8d2882a8edb8 ("mm,memory_hotplug: remove status_change_nid_normal and update documentation")
Signed-off-by: Hao Li <hao.li@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>