rust: init: make #[pin_data] compatible with conditional compilation of fields

This patch allows one to write
```
#[pin_data]
pub struct Foo {
    #[cfg(CONFIG_BAR)]
    a: Bar,
    #[cfg(not(CONFIG_BAR))]
    a: Baz,
}
```
Before, this would result in a compile error, because `#[pin_data]`
would generate two functions named `a` for both fields unconditionally.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-3-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/macros.rs

@@ -962,6 +962,7 @@ macro_rules! __pin_data {
         where $($whr)*
         {
             $(
+                $(#[$($p_attr)*])*
                 $pvis unsafe fn $p_field<E>(
                     self,
                     slot: *mut $p_type,
@@ -971,6 +972,7 @@ macro_rules! __pin_data {
                 }
             )*
             $(
+                $(#[$($attr)*])*
                 $fvis unsafe fn $field<E>(
                     self,
                     slot: *mut $type,