rust: alloc: refactor `Vec::truncate` using `dec_len`

Use `checked_sub` to satisfy the safety requirements of `dec_len` and
replace nearly the whole body of `truncate` with a call to `dec_len`.

Reviewed-by: Andrew Ballance <andrewjballance@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250416-vec-set-len-v4-3-112b222604cd@gmail.com


[ Remove #[expect(unused)] from dec_len(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>