NFS: Ensure we return zero if applications attempt to write zero bytes

A zero byte count direct write request should be a successful no-op, not an
error.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>