[PATCH] rust: bitfield: require Zeroable storage for Zeroable impl

The bitfield! macro implements Zeroable for generated wrapper
types. This assumes the storage type accepts an all-zero bit
pattern. However, the macro accepts any type and does not encode
that requirement.

Add a Zeroable bound to the generated implementation. This prevents
invalid storage types from obtaining an unsound Zeroable implementation.

Fixes: b7b8b4ccdad4 ("rust: extract `bitfield!` macro from `register!`")
Assisted-by: Codex:GPT-5.6 Sol
Signed-off-by: Yilin Chen <redacted>
---
 rust/kernel/bitfield.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/bitfield.rs b/rust/kernel/bitfield.rs
index a0d0894..3dd4dea 100644
--- a/rust/kernel/bitfield.rs
+++ b/rust/kernel/bitfield.rs

@@ -330,8 +330,12 @@ macro_rules! bitfield {
             }
         }
 
-        // SAFETY: `$storage` is `Zeroable` and `$name` is transparent.
-        unsafe impl ::pin_init::Zeroable for $name {}
+        // SAFETY:
+        // - `$storage: Zeroable` guarantees that the all-zero bit pattern is valid.
+        // - `$name` is `repr(transparent)` over `$storage`.
+        unsafe impl ::pin_init::Zeroable for $name
+        where $storage: ::pin_init::Zeroable
+        {}
 
         impl ::core::convert::From<$name> for $storage {
             #[inline(always)]

-- 
2.25.1