rust: ptr: use `match` instead of `unwrap_or_else` for `build_index`

rust: ptr: use `match` instead of `unwrap_or_else` for `build_index`

Use `match` to avoid potential inlining issues of the `unwrap_or_else`
function.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/aeCKlut-88SbNsyW@google.com/
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260602-projection-syntax-rework-v2-2-6989470f5440@garyguo.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>