rust: pin-init: internal: init: simplify Zeroable safety check

rust: pin-init: internal: init: simplify Zeroable safety check

The `Zeroable` type check uses a small dance with a raw pointer to aid
type inference. It turns out that this is not necessary and type
inference is powerful enough to resolve any ambiguity. Thus remove it.

Suggested-by: Gary Guo <gary@garyguo.net>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>