xdrgen: Fix code generated for counted arrays

When an XDR counted array has a maximum element count, xdrgen adds
a bounds check to the encoder or decoder for that type. But in cases
where the .x provides no maximum element count, such as

struct notify4 {
        /* composed from notify_type4 or notify_deviceid_type4 */
        bitmap4         notify_mask;
        notifylist4     notify_vals;
};

struct CB_NOTIFY4args {
        stateid4    cna_stateid;
        nfs_fh4     cna_fh;
        notify4     cna_changes<>;
};

xdrgen is supposed to omit that bounds check. Some of the Jinja2
templates handle that correctly, but a few are incorrect and leave
the bounds check in place with a maximum of zero, which causes
encoding/decoding of that type to fail unconditionally.

Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>