rust: block: fix wrong usage of lockdep API

When allocating `struct gendisk`, `GenDiskBuilder` is using a dynamic
lock class key without registering the key. This is an incorrect use of
the API, which causes a `WARN` trace.

Fix the issue by using a static lock class key, which is more appropriate
for the situation anyway.

Fixes: 3253aba ("rust: block: introduce `kernel::block::mq` module")
Reported-by: Behme Dirk (XC-CP/ESB5) <Dirk.Behme@de.bosch.com>
Closes: https://rust-for-linux.zulipchat.com/#narrow/stream/x/topic/x/near/457090036
Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://lore.kernel.org/r/20240815074519.2684107-3-nmi@metaspace.dk
[ Applied `rustfmt`, reworded slightly and made Zulip link
  a permalink. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: metaspace

rust/kernel/block/mq/gen_disk.rs

@@ -6,8 +6,8 @@
 //! C header: [`include/linux/blk_mq.h`](srctree/include/linux/blk_mq.h)
 
 use crate::block::mq::{raw_writer::RawWriter, Operations, TagSet};
-use crate::error;
 use crate::{bindings, error::from_err_ptr, error::Result, sync::Arc};
+use crate::{error, static_lock_class};
 use core::fmt::{self, Write};
 
 /// A builder for [`GenDisk`].
@@ -93,8 +93,6 @@ impl GenDiskBuilder {
         name: fmt::Arguments<'_>,
         tagset: Arc<TagSet<T>>,
     ) -> Result<GenDisk<T>> {
-        let lock_class_key = crate::sync::LockClassKey::new();
-
         // SAFETY: `bindings::queue_limits` contain only fields that are valid when zeroed.
         let mut lim: bindings::queue_limits = unsafe { core::mem::zeroed() };
 
@@ -110,7 +108,7 @@ impl GenDiskBuilder {
                 tagset.raw_tag_set(),
                 &mut lim,
                 core::ptr::null_mut(),
-                lock_class_key.as_ptr(),
+                static_lock_class!().as_ptr(),
             )
         })?;