vfs: [bugfix] Implement Name() method in WriteFileHandle and ReadFileHandle

Name() method was originally left out and defaulted to the base
class which always returns empty. This trigerred incorrect behavior
in serve nfs where it relied on the Name() of the interafce to figure
out what file it was modifying.

This method is copied from RWFileHandle struct.

Added extra assert in the tests.

vfs/read_write_test.go

@@ -346,6 +346,9 @@ func TestRWFileHandleWriteAt(t *testing.T) {
 		return n
 	}
 
+	// Name
+	assert.Equal(t, "file1", fh.Name())
+
 	// Preconditions
 	assert.Equal(t, int64(0), offset())
 	assert.True(t, fh.opened)