rust: pin-init: examples: mark as #[inline] all From::from()s for Error

There was a recent request in kernel [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 as `#[inline]`.

While in pin-init crate the relevant code is just examples, it
nevertheless does not hurt to use good practice for them.

Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/all/8403c8b7a832b5274743816eb77abfa4@garyguo.net/ [1]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
[ Reworded commit message - Gary ]
Link: https://patch.msgid.link/20260428-pin-init-sync-v1-1-07f9bd3859fb@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

rust/pin-init/examples/error.rs

@@ -11,6 +11,7 @@
 pub struct Error;
 
 impl From<Infallible> for Error {
+    #[inline]
     fn from(e: Infallible) -> Self {
         match e {}
     }
@@ -18,6 +19,7 @@ fn from(e: Infallible) -> Self {
 
 #[cfg(feature = "alloc")]
 impl From<AllocError> for Error {
+    #[inline]
     fn from(_: AllocError) -> Self {
         Self
     }