NFS: Fix attribute cache revalidation

If a NFSv4 client uses the cache_consistency_bitmask in order to
request only information about the change attribute, timestamps and
size, then it has not revalidated all attributes, and hence the
attribute timeout timestamp should not be updated.

Reported-by: Donald Buczek <buczek@molgen.mpg.de>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>