malus: use spawn_blocking (#2804)

Looks like using `spawn` instead of `spawn_blocking` in malus is leading
to a deadlock somehow. I'm not sure why exactly, but switching to
`spawn_blocking` fixes
paritytech/disabling-e2e-tests#1, at least for
`suggest-garbage-candidate` (didn't check other variants).

Maybe my assumption here was wrong:
#2184 (comment).

---------

Co-authored-by: Tsvetomir Dimitrov <tsvetomir@parity.io>

polkadot/node/malus/src/variants/common.rs

@@ -188,7 +188,7 @@ where
 		let _candidate_descriptor = candidate_descriptor.clone();
 		let mut subsystem_sender = subsystem_sender.clone();
 		let (sender, receiver) = std::sync::mpsc::channel();
-		self.spawner.spawn(
+		self.spawner.spawn_blocking(
 			"malus-get-validation-data",
 			Some("malus"),
 			Box::pin(async move {

polkadot/node/malus/src/variants/dispute_finalized_candidates.rs

@@ -95,7 +95,7 @@ where
 
 				let dispute_offset = self.dispute_offset;
 				let mut sender = subsystem_sender.clone();
-				self.spawner.spawn(
+				self.spawner.spawn_blocking(
 					"malus-dispute-finalized-block",
 					Some("malus"),
 					Box::pin(async move {

polkadot/node/malus/src/variants/suggest_garbage_candidate.rs

@@ -113,7 +113,7 @@ where
 					let (sender, receiver) = std::sync::mpsc::channel();
 					let mut new_sender = subsystem_sender.clone();
 					let _candidate = candidate.clone();
-					self.spawner.spawn(
+					self.spawner.spawn_blocking(
 						"malus-get-validation-data",
 						Some("malus"),
 						Box::pin(async move {

prdoc/pr_2804.prdoc

@@ -0,0 +1,9 @@
+title: Fix malus implementation.
+
+doc:
+  - audience: Node Dev
+    description: |
+      The malus implementation is used to test security of Polkadot.
+      It was broken. This fixes it.
+
+crates: [ ]