rust: arc: use `NonNull::new_unchecked`

rust: arc: use `NonNull::new_unchecked`

There is no need to check (and panic on violations of) the safety
requirements on `ForeignOwnable` functions. Avoiding the check is
consistent with the implementation of `ForeignOwnable` for `Box`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20241120-borrow-mut-v6-1-80dbadd00951@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>