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

[Explanation: The dentry is required because in NFSv4, we may
 need to OPEN the file before doing the SETATTR.  (This is
 required if the file size is changed as part of the setattr.)
 Opening the file requires making use of the containing
 directory's inode.]