io_uring/zcrx: remove nth_page() usage within folio

io_uring/zcrx: remove nth_page() usage within folio

Within a folio/compound page, nth_page() is no longer required.  Given
that we call folio_test_partial_kmap()+kmap_local_page(), the code would
already be problematic if the pages would span multiple folios.

So let's just assume that all src pages belong to a single folio/compound
page and can be iterated ordinarily.  The dst page is currently always a
single page, so we're not actually iterating anything.

Link: https://lkml.kernel.org/r/20250901150359.867252-21-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>