mm/sparse.c: set section nid for hot-add memory

In case of NODE_NOT_IN_PAGE_FLAGS is set, we store section's node id in
section_to_node_table[].  While for hot-add memory, this is missed.
Without this information, page_to_nid() may not give the right node id.

BTW, current online_pages works because it leverages nid in
memory_block.  But the granularity of node id should be mem_section
wide.

Link: http://lkml.kernel.org/r/20190618005537.18878-1-richardw.yang@linux.intel.com
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>