rust: alloc: add Box::zeroed()

Add Box::zeroed() for T: Zeroable types.

This allocates with __GFP_ZERO directly, letting the underlying
allocator deal with zeroing out the memory compared to
Box::new(T::zeroed(), flags).

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260505152400.3905096-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/alloc/kbox.rs

@@ -19,6 +19,7 @@
 use crate::fmt;
 use crate::init::InPlaceInit;
 use crate::page::AsPageIter;
+use crate::prelude::*;
 use crate::types::ForeignOwnable;
 use pin_init::{InPlaceWrite, Init, PinInit, ZeroableOption};
 
@@ -256,6 +257,27 @@ pub fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>, A>, AllocError> {
         Ok(Box(ptr.cast(), PhantomData))
     }
 
+    /// Creates a new zero-initialized `Box<T, A>`.
+    ///
+    /// New memory is allocated with `A` and the [`__GFP_ZERO`] flag. The allocation may fail, in
+    /// which case an error is returned. For ZSTs no memory is allocated.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let b = KBox::<[u8; 128]>::zeroed(GFP_KERNEL)?;
+    /// assert_eq!(*b, [0; 128]);
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn zeroed(flags: Flags) -> Result<Self, AllocError>
+    where
+        T: Zeroable,
+    {
+        // SAFETY: `__GFP_ZERO` guarantees the memory is zeroed; `T: Zeroable` guarantees that
+        // all-zeroes is a valid bit pattern for `T`.
+        Ok(unsafe { Self::new_uninit(flags | __GFP_ZERO)?.assume_init() })
+    }
+
     /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then `x` will be
     /// pinned in memory and can't be moved.
     #[inline]