isisd: fix metric calculation of classic lfa backup prefixes

isisd: fix metric calculation of classic lfa backup prefixes

The isis-lfa-topo1 topotest shows backup routes with lower metrics than
the primary ones.

> rt1# show isis route
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::1/128  0       -          -                          -
>  2001:db8:1000::2/128  20      eth-rt2    fe80::d091:eeff:fe09:31cd  -
>  2001:db8:1000::3/128  20      eth-rt3    fe80::946b:d4ff:fe5b:414b  -
>  2001:db8:1000::4/128  20      eth-rt4    fe80::5ced:29ff:feed:59c4  -
> (...)
>
> rt1# show isis route backup
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::2/128  15      eth-rt3    fe80::946b:d4ff:fe5b:414b  -
>                                eth-rt5    fe80::847d:feff:fe74:bdde  -
>                                eth-rt6    fe80::ac8c:dff:feac:8a8d   -
>  2001:db8:1000::3/128  15      eth-rt2    fe80::d091:eeff:fe09:31cd  -
>                                eth-rt5    fe80::847d:feff:fe74:bdde  -
>  2001:db8:1000::4/128  45      eth-rt5    fe80::847d:feff:fe74:bdde  -
> (...)

Backup routes metrics are incorrect because they only take into account
the path metric but not the prefix metric.

Add the prefix metric to the path metric on backup routes.

After the patch with a prefix metric of 10:

> rt1# show isis route backup
> Area 1:
> IS-IS L1 IPv6 routing table:
>
> Area 1:
> IS-IS L1 IPv6 routing table:
>
>  Prefix                Metric  Interface  Nexthop                    Label(s)
>  ------------------------------------------------------------------------------
>  2001:db8:1000::2/128  25      eth-rt3    fe80::8c6f:8aff:fe10:ad0d  -
>                                eth-rt5    fe80::b08e:5cff:fe90:62dd  -
>                                eth-rt6    fe80::4810:47ff:fe81:2b9a  -
>  2001:db8:1000::3/128  25      eth-rt2    fe80::e855:12ff:fe31:5765  -
>                                eth-rt5    fe80::b08e:5cff:fe90:62dd  -
>  2001:db8:1000::4/128  55      eth-rt5    fe80::b08e:5cff:fe90:62dd  -

Fixes: e886416f81 ("isisd: add support for classic LFA (RFC 5286)")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>