hugetlb: handle memory hot-plug events

hugetlb: handle memory hot-plug events

Register per node hstate attributes only for nodes with memory.  As
suggested by David Rientjes.

With Memory Hotplug, memory can be added to a memoryless node and a node
with memory can become memoryless.  Therefore, add a memory on/off-line
notifier callback to [un]register a node's attributes on transition
to/from memoryless state.

N.B.,  Only tested build, boot, libhugetlbfs regression.
       i.e., no memory hotplug testing.

Signed-off-by: Lee Schermerhorn <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Nishanth Aravamudan <[email protected]>
Cc: Adam Litke <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Eric Whitney <[email protected]>
Cc: Christoph Lameter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>