mm/memory_hotplug.c: use pfn_to_nid() instead of page_to_nid(pfn_to_page())

Use "pfn_to_nid(pfn)" instead of "page_to_nid(pfn_to_page(pfn))".

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>