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>