core/network/sync: Request specific fork sync from peers ahead or on par

When making an explicit fork sync request without specifying any peers,
make sure to only request it from the locally known peers that are
either ahead or on a par compared to the block number we are looking
for.

core/network/src/protocol/sync.rs

@@ -465,7 +465,11 @@ impl<B: BlockT> ChainSync<B> {
 				hash, peers,
 			);
 
-			peers = self.peers.iter().map(|(id, _)| id.clone()).collect();
+			peers = self.peers.iter()
+				// Only request blocks from peers who are ahead or on a par.
+				.filter(|(id, peer)| peer.best_number >= number)
+				.map(|(id, _)| id.clone())
+				.collect();
 		} else {
 			debug!(target: "sync", "Explicit sync request for block {:?} with {:?}", hash, peers);
 		}