isisd: fix 'show isis route prefix-sid backup' command

isisd: fix 'show isis route prefix-sid backup' command

It is not possible to dump both backup and prefix-sid
information. Fix this by authorising it.

> rt1# show isis route prefix-sid backup
> Area 1:
> IS-IS paths to level-1 routers that speak IP
> Vertex               Type         Metric Next-Hop             Interface Parent
> rt1
> 10.12.0.0/24         IP internal  0                                     rt1(4)
> 10.13.0.0/24         IP internal  0                                     rt1(4)
> 1.1.1.1/32           IP internal  0                                     rt1(4)
> rt2                  TE-IS        10     rt2                  eth-rt2   rt1(4)
> rt3                  TE-IS        10     rt3                  eth-rt3   rt1(4)
> 10.12.0.0/24         IP TE        20     rt2                  eth-rt2   rt2(4)
> 10.23.0.0/24         IP TE        20     rt2                  eth-rt2   rt2(4)
>                                          rt3                  eth-rt3   rt3(4)
> 2.2.2.2/32           IP TE        20     rt2                  eth-rt2   rt2(4)
> 10.13.0.0/24         IP TE        20     rt3                  eth-rt3   rt3(4)
> 3.3.3.3/32           IP TE        20     rt3                  eth-rt3   rt3(4)
>
> IS-IS L1 IPv4 routing table:
>
> IS-IS paths to level-1 routers that speak IPv6
> Vertex               Type         Metric Next-Hop             Interface Parent
> rt1
> 2001:db8:1000::1/128 IP6 internal 0                                     rt1(4)
> rt2                  TE-IS        10     rt2                  eth-rt2   rt1(4)
> rt3                  TE-IS        10     rt3                  eth-rt3   rt1(4)
> 2001:db8:1000::2/128 IP6 internal 20     rt2                  eth-rt2   rt2(4)
> 2001:db8:1000::3/128 IP6 internal 20     rt3                  eth-rt3   rt3(4)
>
> IS-IS L1 IPv6 routing table:
>

Fixes: d47d6089e06c ("isisd: refactor handling of SR Prefix-SIDs")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>