rust: pin-init: add references to previously initialized fields

After initializing a field in an initializer macro, create a variable
holding a reference that points at that field. The type is either
`Pin<&mut T>` or `&mut T` depending on the field's structural pinning
kind.

[ Applied fixes to devres and rust_driver_pci sample - Benno]
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Benno Lossin <lossin@kernel.org>

samples/rust/rust_driver_pci.rs

@@ -78,8 +78,8 @@ impl pci::Driver for SampleDriver {
 
         let drvdata = KBox::pin_init(
             try_pin_init!(Self {
-                pdev: pdev.into(),
                 bar <- pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci")),
+                pdev: pdev.into(),
                 index: *info,
             }),
             GFP_KERNEL,