Add mutex logging

internal/model/deviceactivity.go

@@ -7,9 +7,8 @@
 package model
 
 import (
-	"sync"
-
 	"github.com/syncthing/protocol"
+	"github.com/syncthing/syncthing/internal/sync"
 )
 
 // deviceActivity tracks the number of outstanding requests per device and can
@@ -23,6 +22,7 @@ type deviceActivity struct {
 func newDeviceActivity() *deviceActivity {
 	return &deviceActivity{
 		act: make(map[protocol.DeviceID]int),
+		mut: sync.NewMutex(),
 	}
 }