mm/memory_hotplug: quieting offline operation

On PowerPC, when dymically removing memory from a system we can see in the
console a lot of messages like this:

[  186.575389] Offlined Pages 4096

This message is displayed on each LMB (256MB) removed, which means that we
removing 1TB of memory, this message is displayed 4096 times.

Moving it to DEBUG to not flood the console.

Link: https://lkml.kernel.org/r/20201211150157.91399-1-ldufour@linux.ibm.com


Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>
Cc: Scott Cheloha <cheloha@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>