vfs: Don't error a r/w file open without cache; delay error until Read called

If we open a file for r/w without the cache we now always return a
handle and return an error if the file is ever read from.  This fixes
incompatibility with cmount under windows.

vfs/write_test.go

@@ -56,6 +56,15 @@ func TestWriteFileHandleMethods(t *testing.T) {
 	assert.Equal(t, int64(5), fi.Size())
 	assert.Equal(t, "file1", fi.Name())
 
+	// Read
+	var buf = make([]byte, 16)
+	_, err = fh.Read(buf)
+	assert.Equal(t, EPERM, err)
+
+	// ReadAt
+	_, err = fh.ReadAt(buf, 0)
+	assert.Equal(t, EPERM, err)
+
 	// Close
 	assert.NoError(t, fh.Close())