mm/sparse.c: use __nr_to_section(section_nr) to get mem_section

__pfn_to_section is defined as __nr_to_section(pfn_to_section_nr(pfn)).

Since we already get section_nr, it is not necessary to get mem_section
from start_pfn. By doing so, we reduce one redundant operation.

Link: http://lkml.kernel.org/r/20190809010242.29797-1-richardw.yang@linux.intel.com
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Tested-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -877,7 +877,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
 	 */
 	page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);
 
-	ms = __pfn_to_section(start_pfn);
+	ms = __nr_to_section(section_nr);
 	set_section_nid(section_nr, nid);
 	section_mark_present(ms);