fat: make sure to set d_ops in fat_get_parent

fat_get_parent needs to setup the dentry operations, otherwise we might
lose them when the NFS server needs to reconnect out of cache inodes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>