rust: sync: introduce LockedBy

This allows us to have data protected by a lock despite not being
wrapped by it. Access is granted by providing evidence that the lock is
held by the caller.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230411054543.21278-13-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/sync/lock.rs

@@ -74,7 +74,7 @@ pub struct Lock<T: ?Sized, B: Backend> {
     _pin: PhantomPinned,
 
     /// The data protected by the lock.
-    data: UnsafeCell<T>,
+    pub(crate) data: UnsafeCell<T>,
 }
 
 // SAFETY: `Lock` can be transferred across thread boundaries iff the data it protects can.