arm: mm: use __pfn_to_section() to get mem_section

Replace the open-coded '__nr_to_section(pfn_to_section_nr(pfn))' in
pfn_valid() with a more concise call to '__pfn_to_section(pfn)'.

No functional change.
Signed-off-by: NGuixiong Wei <guixiongwei@gmail.com>
Link: https://lore.kernel.org/r/20200430161858.11379-1-guixiongwei@gmail.comSigned-off-by: NWill Deacon <will@kernel.org>

arch/arm64/mm/init.c

@@ -272,7 +272,7 @@ int pfn_valid(unsigned long pfn)
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
 
-	if (!valid_section(__nr_to_section(pfn_to_section_nr(pfn))))
+	if (!valid_section(__pfn_to_section(pfn)))
 		return 0;
 #endif
 	return memblock_is_map_memory(addr);