NFSv4: Simplify the NFSv4 OPEN compound

Get rid of the post-op GETATTR on the directory in order to reduce
the amount of processing done on the server.

The cost is that if we later need to stat() the directory, then we
know that the ctime and mtime are likely to be invalid.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>