Revert "powerpc: Set max_mapnr correctly"

This reverts commit 602946ec.

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: 602946ec ("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