ovl: fix incorrect extent info in metacopy case

In metacopy case, we should use ovl_inode_realdata() instead of
ovl_inode_real() to get real inode which has data, so that
we can get correct information of extentes in ->fiemap operation.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>