rust: str: implement Index for BStr

The `Index` implementation on `BStr` was lost when we switched `BStr` from
a type alias of `[u8]` to a newtype. Add back `Index` by implementing
`Index` for `BStr` when `Index` would be implemented for `[u8]`.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Daniel Gomez <da.gomez@samsung.com>
Link: https://lore.kernel.org/r/20250227-module-params-v3-v8-2-ceeee85d9347@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -114,6 +114,17 @@ impl PartialEq for BStr {
     }
 }
 
+impl<Idx> Index<Idx> for BStr
+where
+    [u8]: Index<Idx, Output = [u8]>,
+{
+    type Output = Self;
+
+    fn index(&self, index: Idx) -> &Self::Output {
+        BStr::from_bytes(&self.0[index])
+    }
+}
+
 /// Creates a new [`BStr`] from a string literal.
 ///
 /// `b_str!` converts the supplied string literal to byte string, so non-ASCII