mm/mm/percpu-km: drop nth_page() usage within single allocation

mm/mm/percpu-km: drop nth_page() usage within single allocation

We're allocating a higher-order page from the buddy.  For these pages
(that are guaranteed to not exceed a single memory section) there is no
need to use nth_page().

Link: https://lkml.kernel.org/r/20250901150359.867252-15-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>