rust: ptr: add const_align_up()

rust: ptr: add const_align_up()

Add const_align_up() to kernel::ptr as the const-compatible equivalent
of Alignable::align_up().

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Suggested-by: Gary Guo <gary@garyguo.net>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260326013902.588242-17-jhubbard@nvidia.com
[ Adjusted imports style. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>