rust: dma: generalize BinaryWriter impl for Coherent<T>

Generalize the BinaryWriter implementation from Coherent<[u8]> to
Coherent<T> where T: KnownSize + AsBytes + ?Sized. The implementation
only uses size() and write_dma(), neither of which depends on the
inner type being a byte slice.

This allows any Coherent allocation with an AsBytes inner type to be
exposed as a debugfs binary file.

Acked-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260325003921.3420-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

rust/kernel/dma.rs

@@ -885,7 +885,7 @@ unsafe impl<T: KnownSize + Send + ?Sized> Send for Coherent<T> {}
 // The safe methods only return metadata or raw pointers whose use requires `unsafe`.
 unsafe impl<T: KnownSize + ?Sized + AsBytes + FromBytes + Sync> Sync for Coherent<T> {}
 
-impl debugfs::BinaryWriter for Coherent<[u8]> {
+impl<T: KnownSize + AsBytes + ?Sized> debugfs::BinaryWriter for Coherent<T> {
     fn write_to_slice(
         &self,
         writer: &mut UserSliceWriter,