mm/page_alloc: use deferred_pages_enabled() wherever applicable

Instead of directly accessing static deferred_pages, replace such
instances with the helper deferred_pages_enabled().  No functional change
is intended.

Link: https://lkml.kernel.org/r/20230105082506.241529-1-anshuman.khandual@arm.com


Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>