mm: constify page_is_pfmemalloc() argument

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit 1d7bab6a
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1d7bab6a94458e959f3f55788fd50ddc7d97403b



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The function only tests for page->index, so its argument should be
const.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>