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 f

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 <[email protected]>
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: [email protected]
[ Adjust mentioned commit to the one from the kernel. - Benno ]
Signed-off-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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rust: pin-init: add `std` and `alloc` support from the user-space version

To synchronize the kernel's version of pin-init with the user-space
version, introduce support for `std` and `alloc`. While

rust: pin-init: add `std` and `alloc` support from the user-space version

To synchronize the kernel's version of pin-init with the user-space
version, introduce support for `std` and `alloc`. While the kernel uses
neither, the user-space version has to support both. Thus include the
required `#[cfg]`s and additional code.

Signed-off-by: Benno Lossin <[email protected]>
Reviewed-by: Fiona Behrens <[email protected]>
Tested-by: Andreas Hindborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Undo the temporary `--extern force:alloc` since now we have contents
for `alloc` here. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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