arm64: mm: fix incorrect assignment of 'max_mapnr'

Although we don't actually make use of the 'max_mapnr' global variable,
we do set it to a junk value for !CONFIG_FLATMEM configurations that
leave mem_map uninitialised.

To avoid somebody tripping over this in future, set 'max_mapnr' using
max_pfn, which is calculated directly from the memblock information.
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Muchun Song <smuchun@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/init.c

@@ -535,7 +535,7 @@ void __init mem_init(void)
 	else
 		swiotlb_force = SWIOTLB_NO_FORCE;
 
-	set_max_mapnr(pfn_to_page(max_pfn) - mem_map);
+	set_max_mapnr(max_pfn - PHYS_PFN_OFFSET);
 
 #ifndef CONFIG_SPARSEMEM_VMEMMAP
 	free_unused_memmap();