mm/page_alloc: remove ifdefs from pindex helpers

mm/page_alloc: remove ifdefs from pindex helpers

The ifdefs are not technically needed here, everything used here is
always defined.

Switching to IS_ENABLED() makes the code a bit less tiresome to read.

Link: https://lore.kernel.org/20260513-page_alloc-unmapped-prep-v1-4-dacdf5402be8@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>