rust: alloc: replace `Vec::set_len` with `inc_len`

Rename `set_len` to `inc_len` and simplify its safety contract.

Note that the usage in `CString::try_from_fmt` remains correct as the
receiver is known to have `len == 0`.

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-v4-4-112b222604cd@gmail.com


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