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>

fs/nfs/direct.c

@@ -890,6 +890,8 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, const struct iovec *iov,
 	retval = generic_write_checks(file, &pos, &count, 0);
 	if (retval)
 		goto out;
+	if (!count)
+		goto out;	/* return 0 */
 
 	retval = -EINVAL;
 	if ((ssize_t) count < 0)