rust: sync: replace kernel::c_str! with C-Strings

C-String literals were added in Rust 1.77. Replace instances of
`kernel::c_str!` with C-String literals where possible.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251117-core-cstr-cstrings-v4-1-924886ad9f75@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/sync.rs

@@ -48,7 +48,6 @@ impl LockClassKey {
     ///
     /// # Examples
     /// ```
-    /// # use kernel::c_str;
     /// # use kernel::alloc::KBox;
     /// # use kernel::types::ForeignOwnable;
     /// # use kernel::sync::{LockClassKey, SpinLock};
@@ -60,7 +59,7 @@ impl LockClassKey {
     /// {
     ///     stack_pin_init!(let num: SpinLock<u32> = SpinLock::new(
     ///         0,
-    ///         c_str!("my_spinlock"),
+    ///         c"my_spinlock",
     ///         // SAFETY: `key_ptr` is returned by the above `into_foreign()`, whose
     ///         // `from_foreign()` has not yet been called.
     ///         unsafe { <Pin<KBox<LockClassKey>> as ForeignOwnable>::borrow(key_ptr) }