mm/memory_hotplug: activate node before adding new memory blocks

The sysfs attributes for memory blocks require the node ID to be set and
initialized, so move the node activation before adding new memory blocks. 
This also has the nice side effect that the BUG_ON() can be converted into
a WARN_ON() as we now can handle registration errors.

Link: https://lkml.kernel.org/r/20250729064637.51662-3-hare@kernel.org


Fixes: b9ff0360 ("mm/memory_hotplug.c: make add_memory_resource use __try_online_node")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Donet Tom <donettom@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>