fs: don't log error failing MvChild

Document MvChild failure condition.

Change-Id: I928335eb79c73bdedb14c9f7544c3b15e3862af8

fs/bridge.go

@@ -510,9 +510,8 @@ func (b *rawBridge) Rename(cancel <-chan struct{}, input *fuse.RenameIn, oldName
 			if input.Flags&RENAME_EXCHANGE != 0 {
 				p1.ExchangeChild(oldName, p2, newName)
 			} else {
-				if ok := p1.MvChild(oldName, p2, newName, true); !ok {
-					log.Println("MvChild failed")
-				}
+				// MvChild cannot fail with overwrite=true.
+				_ = p1.MvChild(oldName, p2, newName, true)
 			}
 		}
 		return errnoToStatus(errno)

fs/inode.go

@@ -574,7 +574,8 @@ retry:
 }
 
 // MvChild executes a rename. If overwrite is set, a child at the
-// destination will be overwritten, should it exist.
+// destination will be overwritten, should it exist. It returns false
+// if 'overwrite' is false, and the destination exists.
 func (n *Inode) MvChild(old string, newParent *Inode, newName string, overwrite bool) bool {
 	if len(newName) == 0 {
 		log.Panicf("empty newName for MvChild")