bgpd: fix AIGP calculation in route advertisement (backport #17168)

[mergify]

Currently the AIGP is always incremented when a route with the
attribute is advertised. That is incorrect when the nexthop is
unchanged, as is commonly the case in route reflection.

Adjust the AIGP for propagation only when the nexthop is set
to ourselves.

---

This is an automatic backport of pull request #17168 done by Mergify.

[mergify]

Cherry-pick of fc82d77 has failed:

On branch mergify/bp/stable/9.1/pr-17168
Your branch is ahead of 'origin/stable/9.1' by 1 commit.
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You are currently cherry-picking commit fc82d7750f.
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Changes to be committed:
	modified:   bgpd/bgp_bmp.c
	modified:   bgpd/bgp_route.c
	modified:   bgpd/bgp_updgrp_packet.c

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

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