BACKPORT: mm: page_alloc: group fallback functions together

The way the fallback rules are spread out makes them hard to follow.  Move
the functions next to each other at least.

Link: https://lkml.kernel.org/r/20250225001023.1494422-4-hannes@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Bug: 420836317
(cherry picked from commit a4138a27)
[
1. In the original patch of the find_suitable_fallback function, replace MIGRATE_PCPTYPES with MIGRATE_FALLBACKS.;
2. Keep the hook function in the reserve_highatomic_pageblock and unreserve_highatomic_pageblock functions.
]
Change-Id: I069e8dd7f8b009c686daef4459f9f1452b3f4c2c
Signed-off-by: yipeng xiang <yipengxiang@honor.corp-partner.google.com>