zebra: fix missing kernel routes (backport #17326)

[mergify]

The rib_update_handle_kernel_route_down_possibility() didn't consider the kernel routes ( blackhole ) without interface. When some other interfaces are down, these kernel routes will be wrongly removed.

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This is an automatic backport of pull request #17326 done by Mergify.

[mergify]

Cherry-pick of 44a82da has failed:

On branch mergify/bp/stable/10.0/pr-17326
Your branch is up to date with 'origin/stable/10.0'.

You are currently cherry-picking commit 44a82da405.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   zebra/zebra_rib.c

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 0073a87 has failed:

On branch mergify/bp/stable/10.0/pr-17326
Your branch is ahead of 'origin/stable/10.0' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 0073a870d1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   tests/topotests/zebra_multiple_connected/r1/ip_route_kernel_blackhole.json

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/topotests/zebra_multiple_connected/test_zebra_multiple_connected.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally