This patch changes the interface of the ->readdir() nfs_rpc_op
so that its first argument is a dentry instead of an inode.

[Explanation: The dentry is required because in NFSv4, we need
 to make use of the _parent_ directory's inode.  This is because
 NFSv4 servers no longer return an entry for ".." in the READDIR
 response, so the client kernel needs to fake this entry, inode
 number and all.]