memory hotplug: fix comments when adding section

Here, pfn_to_node should be page_to_nid.

Link: http://lkml.kernel.org/r/1510735205-22540-1-git-send-email-fan.du@intel.com


Signed-off-by: Fan Du <fan.du@intel.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>