rust: alloc: vmalloc: implement Vmalloc::to_page()

Implement an abstraction of vmalloc_to_page() for subsequent use in the
AsPageIter implementation of VBox and VVec.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250820145434.94745-3-dakr@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>