mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()

mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()

There is the concern that unpin_user_page_range_dirty_lock() might do some
weird merging of PFN ranges -- either now or in the future -- such that
PFN range is contiguous but the page range might not be.

Let's sanity-check for that and drop the nth_page() usage.

Link: https://lkml.kernel.org/r/20250901150359.867252-35-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>