mm/page_alloc: remove unneeded variable base

Since commit 5d0a661d ("mm/page_alloc: use only one PCP list for
THP-sized allocations"), local variable base is just as same as order.  So
remove it.  No functional change intended.

Link: https://lkml.kernel.org/r/20230803114934.693989-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>