mm, page_alloc: cache pageset high and batch in struct zone

All per-cpu pagesets for a zone use the same high and batch values, that
are duplicated there just for performance (locality) reasons.  This patch
adds the same variables also to struct zone as a shared copy.

This will be useful later for making possible to disable pcplists
temporarily by setting high value to 0, while remembering the values for
restoring them later.  But we can also immediately benefit from not
updating pagesets of all possible cpus in case the newly recalculated
values (after sysctl change or memory online/offline) are actually
unchanged from the previous ones.

Link: https://lkml.kernel.org/r/20201111092812.11329-6-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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