NFSv4: Fix an embarassing typo in encode_attrs()

Apparently, we have never been able to set the atime correctly from the
NFSv4 client.

Reported-by: 小倉一夫 <ka-ogura@bd6.so-net.ne.jp>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org