mm: hugetlb: eliminate memory-less nodes handling

The memory-notify-based approach aims to handle meory-less nodes, however,
it just adds the complexity of code as pointed by David in thread [1]. 
The handling of memory-less nodes is introduced by commit 4faf8d950ec4
("hugetlb: handle memory hot-plug events").  >From its commit message, we
cannot find any necessity of handling this case.  So, we can simply
register/unregister sysfs entries in register_node/unregister_node to
simlify the code.

BTW, hotplug callback added because in hugetlb_register_all_nodes() we
register sysfs nodes only for N_MEMORY nodes, seeing commit 9b5e5d0fdc91,
which said it was a preparation for handling memory-less nodes via memory
hotplug.  Since we want to remove memory hotplug, so make sure we only
register per-node sysfs for online (N_ONLINE) nodes in
hugetlb_register_all_nodes().

https://lore.kernel.org/linux-mm/[email protected]/ [1]
Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: David Hildenbrand <[email protected]>
Signed-off-by: Muchun Song <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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