zebra: Correctly report metrics

zebra: Correctly report metrics

Report the routes metric in IPFORWARDMETRIC1 and return
-1 for the other metrics as required by the IP-FORWARD-MIB.

inetCidrRouteMetric2 OBJECT-TYPE
    SYNTAX     Integer32
    MAX-ACCESS read-create
    STATUS     current
    DESCRIPTION
           "An alternate routing metric for this route.  The
            semantics of this metric are determined by the routing-
            protocol specified in the route's inetCidrRouteProto
            value.  If this metric is not used, its value should be
            set to -1."
    DEFVAL { -1 }
    ::= { inetCidrRouteEntry 13 }

I've included metric2 but it's the same for all of them.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit e41ae0acc1940b568def5018efad3df019023f85)