mm/memory_hotplug: enforce section granularity when onlining/offlining

Already two people (including me) tried to offline subsections, because
the function looks like it can deal with it.  But we really can only
online/offline full sections that are properly aligned (e.g., we can only
mark full sections online/offline via SECTION_IS_ONLINE).

Add a simple safety net to document the restriction now.  Current users
(core and powernv/memtrace) respect these restrictions.

Signed-off-by: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Pankaj Gupta <[email protected]>
Cc: Charan Teja Reddy <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Tony Luck <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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