Revert "powerpc: Set max_mapnr correctly"

This reverts commit 602946e.

If CONFIG_HIGHMEM is enabled, no highmem will be added with max_mapnr
set to max_low_pfn, see mem_init():

  for (pfn = highmem_mapnr; pfn < max_mapnr; ++pfn) {
        ...
        free_highmem_page();
  }

Now that virt_addr_valid() has been fixed in the previous commit, we can
revert the change to max_mapnr.

Fixes: 602946e ("powerpc: Set max_mapnr correctly")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reported-by: Erhard F. <erhard_f@mailbox.org>
[mpe: Update change log to reflect series reordering]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220406145802.538416-2-mpe@ellerman.id.au

Author: Kefeng Wang

arch/powerpc/mm/mem.c

@@ -255,7 +255,7 @@ void __init mem_init(void)
 #endif
 
 	high_memory = (void *) __va(max_low_pfn * PAGE_SIZE);
-	set_max_mapnr(max_low_pfn);
+	set_max_mapnr(max_pfn);
 
 	kasan_late_init();