binder: Fix selftest page indexing

The binder allocator selftest was only checking the last page of buffers
that ended on a page boundary. Correct the page indexing to account for
buffers that are not page-aligned.

Signed-off-by: Tiffany Yang <ynaffit@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20250714185321.2417234-2-ynaffit@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>