Fixed a bug in forgetfs.

This manifested as ForgetFSTest.MkDir pacicking now and then in a manner
like this:

    https://gist.githubusercontent.com/jacobsa/2d12150beca1043f71b0/raw/28e0a01a9276799ad820970a05b522367222c331/gistfile1.txt

samples/forgetfs/forget_fs.go

@@ -66,6 +66,13 @@ func NewFileSystem() (fs *ForgetFS) {
 	// The root inode starts with a lookup count of one.
 	impl.inodes[cannedID_Root].IncrementLookupCount()
 
+	// The canned inodes are supposed to be stable from the user's point of view,
+	// so we should allow them to be looked up at any point even if the kernel
+	// has balanced its lookups with its forgets. Ensure that they never go to
+	// zero until the file system is destroyed.
+	impl.inodes[cannedID_Foo].IncrementLookupCount()
+	impl.inodes[cannedID_Bar].IncrementLookupCount()
+
 	// Set up the mutex.
 	impl.mu = syncutil.NewInvariantMutex(impl.checkInvariants)