mm/memory_hotplug.c: use __pfn_to_section() instead of open-coding

Use __pfn_to_section() API instead of open-coding for better code
readability.

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand <david@redhat.com>
Link: http://lkml.kernel.org/r/1584345134-16671-1-git-send-email-qiwuchen55@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -495,7 +495,7 @@ static void __remove_section(unsigned long pfn, unsigned long nr_pages,
 			     unsigned long map_offset,
 			     struct vmem_altmap *altmap)
 {
-	struct mem_section *ms = __nr_to_section(pfn_to_section_nr(pfn));
+	struct mem_section *ms = __pfn_to_section(pfn);
 
 	if (WARN_ON_ONCE(!valid_section(ms)))
 		return;