vfs: fix fatal error: sync: unlock of unlocked mutex in panics

Before this change a panic could be overwritten with the message

    fatal error: sync: unlock of unlocked mutex

This was because we temporarily unlocked the mutex, but failed to lock
it again if there was a panic.

This is code is never the cause of an error but it masks the
underlying error by overwriting the panic cause.

See: https://forum.rclone.org/t/serve-webdav-is-crashing-fatal-error-sync-unlock-of-unlocked-mutex/46300

vfs/vfscache/item.go

@@ -573,6 +573,15 @@ func (item *Item) open(o fs.Object) (err error) {
 	return err
 }
 
+// Calls f with mu unlocked, re-locking mu if a panic is raised
+//
+// mu must be locked when calling this function
+func unlockMutexForCall(mu *sync.Mutex, f func()) {
+	mu.Unlock()
+	defer mu.Lock()
+	f()
+}
+
 // Store stores the local cache file to the remote object, returning
 // the new remote object. objOld is the old object if known.
 //
@@ -589,9 +598,9 @@ func (item *Item) _store(ctx context.Context, storeFn StoreFn) (err error) {
 	// Object has disappeared if cacheObj == nil
 	if cacheObj != nil {
 		o, name := item.o, item.name
-		item.mu.Unlock()
-		o, err := operations.Copy(ctx, item.c.fremote, o, name, cacheObj)
-		item.mu.Lock()
+		unlockMutexForCall(&item.mu, func() {
+			o, err = operations.Copy(ctx, item.c.fremote, o, name, cacheObj)
+		})
 		if err != nil {
 			if errors.Is(err, fs.ErrorCantUploadEmptyFiles) {
 				fs.Errorf(name, "Writeback failed: %v", err)