arm64:mm: initialize max_mapnr using function set_max_mapnr

Initializing max_mapnr using set_max_mapnr() helper function instead
of direct reference. Also not adding PHYS_PFN_OFFSET to max_pfn,
since it already contains it.
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@caviumnetworks.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/init.c

@@ -256,7 +256,7 @@ static void __init free_unused_memmap(void)
  */
 void __init mem_init(void)
 {
-	max_mapnr   = pfn_to_page(max_pfn + PHYS_PFN_OFFSET) - mem_map;
+	set_max_mapnr(pfn_to_page(max_pfn) - mem_map);
 
 #ifndef CONFIG_SPARSEMEM_VMEMMAP
 	free_unused_memmap();