fix: linux route metric

[VladoLavor]

• the metric change causes the linux route to recreate instead of replacing
  (which led to the creation of a new route instead)
• for IPv6, the metric 0 and 1024 is considered equal (based on how Linux
  treats it)

Signed-off-by: Vladimir Lavor vlavor@cisco.com

[mhalaj1]

When examining this example, I found a bug (unrelated to this PR): VPP agent can not configure IPv6 route with scope set to LINK.
How to reproduce the bug:
Make a copy of the resync method and name it resync2. In the copied method, replace the lines

LinuxRoute(routeResync()).
LinuxRoute(routeResyncIPv6()).

with

LinuxRoute(routeModified()).
LinuxRoute(routeModifiedIPv6()).

Add the following code at the end of AfterInit method (at line 129):

ctx, p.cancelResync = context.WithCancel(context.Background())
p.wg.Add(1)
go p.resync2(ctx, *timeout*3)

Then build and run the example.
The log of the last resync transaction contains the following:

  * executed operations (2021-11-30 17:24:49.56 +0100 CET -> 2021-11-30 17:24:49.58 +0100 CET, dur: 21ms):
      1. UPDATE:
          - key: config/linux/l3/v2/route/aaa1::/64/linux-tap2
          - prev-value: { outgoing_interface:"linux-tap2"  scope:GLOBAL  dst_network:"aaa1::/64"  metric:500 } 
          - new-value: { outgoing_interface:"linux-tap2"  scope:LINK  dst_network:"aaa1::/64"  metric:500 } 
      2. CREATE [PROPERTY WAS-MISSING]:
          - key: linux/link-local-route/linux-tap2/dest-address/aaa1::/64
          - value: <EMPTY> 

There should be no executed operations.
This shows that the LINK scope has not been configured.