core/txpool/locals: fix data race on journal writer

The journal.setupWriter and journal.close calls in TxTracker.loop
are not protected by tracker.mu, while recheck and TrackAll access
journal.writer under the same mutex. This causes a data race
between the loop goroutine and callers of recheck/TrackAll.

Hold tracker.mu around setupWriter and close in loop to
synchronize access to journal.writer.

Fixes a race detected by go test -race:

  Write at journal.setupWriter (journal.go:134)
    by goroutine running TxTracker.loop

  Previous read at journal.rotate (journal.go:154)
    by goroutine running TxTracker.recheck

core/txpool/locals/tx_tracker.go

@@ -195,12 +195,21 @@ func (tracker *TxTracker) loop() {
 			return nil
 		})
 
-		// Setup the writer for the upcoming transactions
-		if err := tracker.journal.setupWriter(); err != nil {
+		// Setup the writer for the upcoming transactions.
+		// Hold the mutex to avoid racing with recheck/TrackAll
+		// which also access journal.writer.
+		tracker.mu.Lock()
+		err := tracker.journal.setupWriter()
+		tracker.mu.Unlock()
+		if err != nil {
 			log.Error("Failed to setup the journal writer", "err", err)
 			return
 		}
-		defer tracker.journal.close()
+		defer func() {
+			tracker.mu.Lock()
+			tracker.journal.close()
+			tracker.mu.Unlock()
+		}()
 	}
 	var (
 		lastJournal = time.Now()