rust: bitfield: mark Debug impl as #[inline]

A `Debug` impl is for debugging and is normally not used, and therefore
should ideally not be code-generated unless used. However, Rust has no way
of knowing if a dependent crate is going to use the trait impl or not, so
unless it is marked as `#[inline]`, it will be code-generated in the
defining crate (as it is not generic).

Mark the impl generated by bitfield macro `#[inline]`, so they do not stay
in the binary unless used.

This reduces nova-core.o .text by 17% (from 151922 bytes to 125676 bytes).

Signed-off-by: Gary Guo <gary@garyguo.net>
Fixes: b7b8b4ccda ("rust: extract `bitfield!` macro from `register!`")
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260611190555.2298991-1-gary@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/bitfield.rs

@@ -535,6 +535,7 @@ const fn [<__with_ $field>](
     // `Debug` implementation.
     (@debug $name:ident { $($field:ident;)* }) => {
         impl ::kernel::fmt::Debug for $name {
+            #[inline]
             fn fmt(&self, f: &mut ::kernel::fmt::Formatter<'_>) -> ::kernel::fmt::Result {
                 f.debug_struct(stringify!($name))
                     .field("<raw>", &::kernel::prelude::fmt!("{:#x}", self.inner))