rust: sync: add #[must_use] to Lock::try_lock

The `Lock::try_lock` function returns an `Option<Guard<...>>`, but it
currently does not issue a warning if the return value is unused.
To avoid potential bugs, the `#[must_use]` annotation is added to ensure proper usage.

Note that `T` is `#[must_use]` but `Option<T>` is not.
For more context, see: rust-lang/rust#71368.

Suggested-by: Alice Ryhl <[email protected]>
Link: Rust-for-Linux#1133
Signed-off-by: Jason Devers <[email protected]>

Author: hz2

rust/kernel/sync/lock.rs

@@ -147,6 +147,8 @@ impl<T: ?Sized, B: Backend> Lock<T, B> {
     /// Tries to acquire the lock.
     ///
     /// Returns a guard that can be used to access the data protected by the lock if successful.
+    // Note that `T` is `#[must_use]` but `Option<T>` is not.
+    #[must_use = "if unused, the lock will be immediately unlocked"]
     pub fn try_lock(&self) -> Option<Guard<'_, T, B>> {
         // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
         // that `init` was called.