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.comSigned-off-by: NFan Du <fan.du@intel.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -265,7 +265,7 @@ static int __meminit __add_section(int nid, unsigned long phys_start_pfn,
 	/*
 	 * Make all the pages reserved so that nobody will stumble over half
 	 * initialized state.
-	 * FIXME: We also have to associate it with a node because pfn_to_node
+	 * FIXME: We also have to associate it with a node because page_to_nid
 	 * relies on having page with the proper node.
 	 */
 	for (i = 0; i < PAGES_PER_SECTION; i++) {