T5722: Failover route add option onlink

onlink pretend that the nexthop is directly attached to this link,
even if it does not match any interface prefix.

Useful when gateway not in the same interface network

set interfaces ethernet eth0 vif 10 address '10.20.30.1/32'
set protocols static route 10.20.30.0/32 interface eth0.10
set protocols failover route 192.0.2.11/32 next-hop 10.20.30.0 onlink

vyos@r4# sudo ip route add 192.0.2.111/32 via 10.20.30.0 dev eth0.10 metric 1 proto failover
Error: Nexthop has invalid gateway.
[edit]
vyos@r4#
[edit]
vyos@r4# sudo ip route add 192.0.2.111/32 via 10.20.30.0 dev eth0.10 onlink metric 1 proto failover
[edit]
vyos@r4#