rust: page: implement BorrowedPage

rust: page: implement BorrowedPage

Currently, a Page always owns the underlying struct page.

However, sometimes a struct page may be owned by some other entity, e.g.
a vmalloc allocation.

Hence, introduce BorrowedPage to support such cases, until the Ownable
solution [1] lands.

This is required by the scatterlist abstractions.

Acked-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/rust-for-linux/ZnCzLIly3DRK2eab@boqun-archlinux/
Link: https://lore.kernel.org/r/20250820145434.94745-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>