mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves

GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a vague
description.  In preparation for the removal of GFP_ATOMIC redefine
__GFP_ATOMIC to simply mean non-blocking and renaming ALLOC_HARDER to
ALLOC_NON_BLOCK accordingly.  __GFP_HIGH is required for access to
reserves but non-blocking is granted more access.  For example, GFP_NOWAIT
is non-blocking but has no special access to reserves.  A __GFP_NOFAIL
blocking allocation is granted access similar to __GFP_HIGH if the only
alternative is an OOM kill.

Link: https://lkml.kernel.org/r/20230113111217.14134-6-mgorman@techsingularity.net


Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>