mm: use __pfn_to_section() instead of open coding it

It is defined in the same file just a few lines above.

Link: https://lkml.kernel.org/r/4598487.Rc0NezkW7i@mobilepool36.emlix.comSigned-off-by: NRolf Eike Beer <eb@emlix.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -1481,7 +1481,7 @@ static inline int pfn_valid(unsigned long pfn)
 
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
-	ms = __nr_to_section(pfn_to_section_nr(pfn));
+	ms = __pfn_to_section(pfn);
 	if (!valid_section(ms))
 		return 0;
 	/*
@@ -1496,7 +1496,7 @@ static inline int pfn_in_present_section(unsigned long pfn)
 {
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
-	return present_section(__nr_to_section(pfn_to_section_nr(pfn)));
+	return present_section(__pfn_to_section(pfn));
 }
 
 static inline unsigned long next_present_section_nr(unsigned long section_nr)