BACKPORT: mm/page_alloc: clarify terminology in migratetype fallback code

Patch series "mm/page_alloc: Some clarifications for migratetype
fallback", v4.

A couple of patches to try and make the code easier to follow.

This patch (of 2):

This code is rather confusing because:

 1. "Steal" is sometimes used to refer to the general concept of
    allocating from a from a block of a fallback migratetype
    (steal_suitable_fallback()) but sometimes it refers specifically to
    converting a whole block's migratetype (can_steal_fallback()).

 2. can_steal_fallback() sounds as though it's answering the question "am
    I functionally permitted to allocate from that other type" but in
    fact it is encoding a heuristic preference.

 3. The same piece of data has different names in different places:
    can_steal vs whole_block. This reinforces point 2 because it looks
    like the different names reflect a shift in intent from "am I
    allowed to steal" to "do I want to steal", but no such shift exists.

Fix 1. by avoiding the term "steal" in ambiguous contexts. Start using
the term "claim" to refer to the special case of stealing the entire
block.

Fix 2. by using "should" instead of "can", and also rename its
parameters and add some commentary to make it more explicit what they
mean.

Fix 3. by adopting the new "claim" terminology universally for this
set of variables.

Link: https://lkml.kernel.org/r/20250228-clarify-steal-v4-0-cb2ef1a4e610@google.com
Link: https://lkml.kernel.org/r/20250228-clarify-steal-v4-1-cb2ef1a4e610@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Bug: 420836317
Bug: 429525621
(cherry picked from commit e47f1f56)
[In the original patch of the find_suitable_fallback function, replace MIGRATE_PCPTYPES with MIGRATE_FALLBACKS.;]
Change-Id: I8f1b57aebf308f378f50cd1381f31d249362078e
Signed-off-by: yipeng xiang <yipengxiang@honor.corp-partner.google.com>