This PR fixes the issue that neighbor entries can be installed on devices
where no route to the destination host exists.

`netlink.RouteGetWithOptions` (equivalent to `ip route get`) with oif
returns a route even if the destination is unreachable from the specified interface.
The neighbor entries can be installed on all devices because of this.

`netlink.RouteGetWithOptions` with `FIBMatch` option (equivalent to `fibmatch` flag)
returns full fib lookup matched route. It returns `No route to host`
if the destination is non-routable from the specified device.
This PR adds `FIBMatch` option to avoid installing the unnecessary entries.

Note:

- With IPv6, it returns `Network is unreachable` if the destination
  is unreachable with or without fibmatch.
- With `FIBMatch` option, `netlink.RouteGetWithOptions` returns `MultiPath` field
  if there are multiple paths to the dest. It returns `GW` field without `FIBMatch`.
  See examples below.

```
// FIBMatch: false
netlink.Route{ GW: 8.8.8.250, MultiPath: [] }
netlink.Route{ GW: 9.9.9.250, MultiPath: [] }

// FIBMatch: true
netlink.Route{ GW: <nil>, MultiPath: [{Ifindex: 1218 Weight: 1 Gw: 9.9.9.250 Flags: []},
                                      {Ifindex: 1220 Weight: 1 Gw: 8.8.8.250 Flags: []}]}
```

`ip route get` examples

```
$ ip route
10.0.0.0/24 dev veth1 proto kernel scope link src 10.0.0.1
10.0.1.0/24 dev veth3 proto kernel scope link src 10.0.1.1

$ ping -I veth1 10.0.0.1
PING 10.0.0.1 (10.0.0.1) from 10.0.0.1 veth1: 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=0.066 ms
64 bytes from 10.0.0.1: icmp_seq=2 ttl=64 time=0.056 ms
64 bytes from 10.0.0.1: icmp_seq=3 ttl=64 time=0.057 ms

$ ping -I veth3 10.0.0.1
PING 10.0.0.1 (10.0.0.1) from 10.0.1.1 veth3: 56(84) bytes of data.
From 10.0.1.1 icmp_seq=1 Destination Host Unreachable
From 10.0.1.1 icmp_seq=2 Destination Host Unreachable
From 10.0.1.1 icmp_seq=3 Destination Host Unreachable

$ ip route get 10.0.0.1 oif veth1
local 10.0.0.1 dev lo table local src 10.0.0.1 uid 1000
    cache <local>

// `ip route get` returns a route even if the destination is unreachable
// from the specified interface
$ ip route get 10.0.0.1 oif veth3
10.0.0.1 dev veth3 src 10.0.1.1 uid 1000
    cache

// With fibmatch flag, it returns full fib lookup matched route
$ ip route get fibmatch 10.0.0.1 oif veth1
local 10.0.0.1 dev veth1 proto kernel scope host src 10.0.0.1

$ ip route get fibmatch 10.0.0.1 oif veth3
RTNETLINK answers: No route to host
```

Fixes: cilium#28660

Signed-off-by: Yusuke Suzuki <ysuzuki4112@gmail.com>