rust: pin-init: alloc: restrict `impl ZeroableOption` for `Box` to `T: Sized`

Similar to what was done for `Zeroable<NonNull<T>>` in commit
df27cef15360 ("rust: init: fix `Zeroable` implementation for
`Option<NonNull<T>>` and `Option<KBox<T>>`"), the latest Rust
documentation [1] says it guarantees that `transmute::<_,
Option<T>>([0u8; size_of::<T>()])` is sound and produces
`Option::<T>::None` only in some cases. In particular, it says:

    `Box<U>` (specifically, only `Box<U, Global>`) when `U: Sized`

Thus restrict the `impl` to `Sized`, and use similar wording as in that
commit too.

Link: https://doc.rust-lang.org/stable/std/option/index.html#representation [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/pin-init/pull/32/commits/a6007cf555e5946bcbfafe93a6468c329078acd8
Fixes: 9b2299af3b92 ("rust: pin-init: add `std` and `alloc` support from the user-space version")
Cc: stable@vger.kernel.org
[ Adjust mentioned commit to the one from the kernel. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250407201755.649153-2-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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