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/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Reviewed-by: Brendan Jackman <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: hnaz