rust: kernel: mark as `#[inline]` all `From::from()`s for `Error`

rust: kernel: mark as `#[inline]` all `From::from()`s for `Error`

There was a recent request [1] to mark as `#[inline]` the simple
`From::from()` functions implemented for `Error`.

Thus mark all of the existing

    impl From<...> for Error {
        fn from(err: ...) -> Self {
            ...
        }
    }

functions in the `kernel` crate as `#[inline]`.

Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/all/8403c8b7a832b5274743816eb77abfa4@garyguo.net/
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260326020406.1438210-1-alistair.francis@wdc.com
[ Dropped `projection.rs` since it is in another tree and already marked
  as `inline(always)` and reworded accordingly. Changed Link tag to
  Gary's original message and added Suggested-by. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>