sync: fix deadlock with --track-renames-strategy modtime - fixes #4427

Before this change we could exit the popRenameMap function with the
lock held.

This fixes the problem by defer-ring the unlock.

See: https://forum.rclone.org/t/track-renames-strategy-modtime-doesnt-work/16992

fs/sync/sync.go

@@ -658,6 +658,7 @@ func (s *syncCopyMove) pushRenameMap(hash string, obj fs.Object) {
 // renameMap or returns nil if not found.
 func (s *syncCopyMove) popRenameMap(hash string, src fs.Object) (dst fs.Object) {
 	s.renameMapMu.Lock()
+	defer s.renameMapMu.Unlock()
 	dsts, ok := s.renameMap[hash]
 	if ok && len(dsts) > 0 {
 		// Element to remove
@@ -690,7 +691,6 @@ func (s *syncCopyMove) popRenameMap(hash string, src fs.Object) (dst fs.Object)
 			delete(s.renameMap, hash)
 		}
 	}
-	s.renameMapMu.Unlock()
 	return dst
 }