all: replace RWMutex with Mutex in places where RLock is not used (#21622)

miner/unconfirmed.go

@@ -50,7 +50,7 @@ type unconfirmedBlocks struct {
 	chain  chainRetriever // Blockchain to verify canonical status through
 	depth  uint           // Depth after which to discard previous blocks
 	blocks *ring.Ring     // Block infos to allow canonical chain cross checks
-	lock   sync.RWMutex   // Protects the fields from concurrent access
+	lock   sync.Mutex     // Protects the fields from concurrent access
 }
 
 // newUnconfirmedBlocks returns new data structure to track currently unconfirmed blocks.