bgpd: Intern default-originate attributes to avoid use-after-free

When we receive a default route from a peer and we originate default route
using `neighbor default-originate`, we do not track of struct attr we use,
and when we do `no neighbor default-originate` we withdraw our generated
default route, but we announce default-route from the peer.

After we do this, we unintern aspath (which was used for default-originate),
BUT it was used also for peer's default route we received.

And here we have a use-after-free crash, because bgp_process_main_one()
reaps old paths that are marked as BGP_PATH_REMOVED with aspath->refcnt > 0,
but here it's 0.

```
0 0x55c24bbcd022 in aspath_key_make bgpd/bgp_aspath.c:2070
1 0x55c24b8f1140 in attrhash_key_make bgpd/bgp_attr.c:777
2 0x7f52322e66c9 in hash_release lib/hash.c:220
3 0x55c24b8f6017 in bgp_attr_unintern bgpd/bgp_attr.c:1271
4 0x55c24ba0acaa in bgp_path_info_free_with_caller bgpd/bgp_route.c:283
5 0x55c24ba0a7de in bgp_path_info_unlock bgpd/bgp_route.c:309
6 0x55c24ba0af6d in bgp_path_info_reap bgpd/bgp_route.c:426
7 0x55c24ba17b9a in bgp_process_main_one bgpd/bgp_route.c:3333
8 0x55c24ba18a1d in bgp_process_wq bgpd/bgp_route.c:3425
9 0x7f52323c2cd5 in work_queue_run lib/workqueue.c:282
10 0x7f52323aab92 in thread_call lib/thread.c:2006
11 0x7f5232300dc7 in frr_run lib/libfrr.c:1198
12 0x55c24b8ea792 in main bgpd/bgp_main.c:520
13 0x7f5231c3a082 in __libc_start_main ../csu/libc-start.c:308
14 0x55c24b8ef0bd in _start (/usr/lib/frr/bgpd+0x2c90bd)
    ```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

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