powerpc/pseries/cmm: Use adjust_managed_page_count() insted of totalram_pages_*

adjust_managed_page_count() performs a totalram_pages_add(), but also
adjusts the managed pages of the zone. Let's use that instead, similar
to virtio-balloon. Use it before freeing a page.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191031142933.10779-6-david@redhat.com