rust: error: import `kernel`'s `LayoutError` instead of `core`'s

rust: error: import `kernel`'s `LayoutError` instead of `core`'s

Import the internal (`kernel::alloc`) version of `LayoutError` instead
of the `core::alloc` one.

In particular, this results in switching the type in the existing
`From<LayoutError> for Error` implementation.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Jimmy Ostler <jtostler1@gmail.com>
Link: https://lore.kernel.org/r/fe58a02189e8804a9eabdd01cb1927d4c491d79c.1734674670.git.jtostler1@gmail.com
[ Reworded commit. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>