rust: init: simplify from `map_err` to `inspect_err`

A new complexity lint, `manual_inspect` [1], has been introduced in
the upcoming Rust 1.81 (currently in nightly), which checks for uses of
`map*` which return the original item:

error:
--> rust/kernel/init.rs:846:23
|
846 | (self.1)(val).map_err(|e| {
| ^^^^^^^
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_inspect
= note: `-D clippy::manual-inspect` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::manual_inspect)]`
help: try
|
846 ~ (self.1)(val).inspect_err(|e| {
847 | // SAFETY: `slot` was initialized above.
848 ~ unsafe { core::ptr::drop_in_place(slot) };
|

Thus clean them up.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#/manual_inspect [1]
Tested-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20240709160615.998336-3-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>