memory hotplug: use macro to switch between section and pfn

Use macro section_nr_to_pfn() to switch between section and pfn, instead
of open-coding it.  No semantic changes.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>