af_unix: Unlink scc_entry in unix_del_edge().

Kyle Zeng reported that GC could free a dead SCC partially.

The scenario is as follows:

   1) Create two SCCs:

       X -.   A <-> B
       ^--'

   2) Run the following concurrently:

      2-1) send() sk-B to sk-B from sk-X
      2-2) close() both A and B

At 2-1), there is a small window where unix_add_edges()
publishes a new edge (B <-> B) to GC but its skb is not queued
by skb_queue_tail().

If 2-2) completes before skb_queue_tail() and GC is triggered,
it judges A <-> B as dead, but B is not freed because GC cannot
collect the not-yet-queued skb holding the B <-> B edge.

       X -.   A <-> B -. This edge is visible
       ^--'         ^..'  but skb is not

This itself is not a problem since the next GC run will judge
B as dead as well and free it finally.

       X -.   A <.> B -.
       ^--'         ^--'

However, X's SCC forces the next GC to call unix_walk_scc_fast(),
and it iterates over A through B's scc_entry.

Let's unlink scc_entry before freeing the vertex in unix_del_edge().

Fixes: 4090fa373f ("af_unix: Replace garbage collection algorithm.")
Reported-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Kyle Zeng <kylebot@openai.com>
Fixes: 4090fa373f ("af_unix: Replace garbage collection algorithm.").
Link: https://patch.msgid.link/20260804002155.2233594-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/garbage.c

@@ -186,6 +186,7 @@ static void unix_del_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
 	if (!vertex->out_degree) {
 		edge->predecessor->vertex = NULL;
 		list_move_tail(&vertex->entry, &fpl->vertices);
+		list_del(&vertex->scc_entry);
 	}
 }