isisd: After the router switches IS-IS type several times, the neighbor adjacency...

isisd: After the router switches IS-IS type several times, the neighbor adjacency cannot be established.

1. Router A is configured with "is-type level-1-2", while Router B is configured with "is-type level-1". Only level 1 neighbor entries are present on Router A.
2. After configuring Router B with "is-type level-2-only", both level 1 and level 2 neighbor entries exist on Router A. The state of these entries is UP, and the level 1 neighbor entry is currently aging.
3. Before the level 1 neighbor entry on Router A ages out, configuring Router B with "is-type level-1", both level 1 and level 2 neighbor entries exist on Router A. The level 2 neighbor entry is UP and will age out normally. However, the level 1 neighbor entry remains in the Initializing state, preventing the establishment of level 1 neighbor adjacency between Router A and Router B.

When the adjacency type of the link is switched in function isis_circuit_is_type_set, the function circuit_resign_level() is called to delete the old level's circuit->u.bc.lan_neighs linked list. If the old level is not level-1-2, the function circuit_commence_level() is called to create a new level's circuit->u.bc.lan_neighs linked list, but neither of these functions handle the circuit->u.bc.adjdb linked list. This leads to a situation where upon receiving hello packets again before the circuit->u.bc.adjdb linked list entries age out, the circuit->u.bc.lan_neighs linked list is not constructed based on the circuit->u.bc.adjdb linked list. As a result, the hello packets sent will consistently lack an SNPA, causing the neighbor to remain unable to establish an adjacency upon receiving the hello packets.

Signed-off-by: zhou-run <166502045+zhou-run@users.noreply.github.com>