mm/memory_hotplug: document why shuffle_zone() is relevant

It's not completely obvious why we have to shuffle the complete zone -
introduced in commit e900a918

 ("mm: shuffle initial free memory to
improve memory-side-cache utilization") - because some sort of shuffling
is already performed when onlining pages via __free_one_page(), placing
MAX_ORDER-1 pages either to the head or the tail of the freelist.  Let's
document why we have to shuffle the complete zone when exposing larger,
contiguous physical memory areas to the buddy.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Link: http://lkml.kernel.org/r/20200624094741.9918-3-david@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>