af_unix: Simplify GC state.

GC manages its state by two variables, unix_graph_maybe_cyclic
and unix_graph_grouped, both of which are set to false in the
initial state.

When an AF_UNIX socket is passed to an in-flight AF_UNIX socket,
unix_update_graph() sets unix_graph_maybe_cyclic to true and
unix_graph_grouped to false, making the next GC invocation call
unix_walk_scc() to group SCCs.

Once unix_walk_scc() finishes, sockets in the same SCC are linked
via vertex->scc_entry.  Then, unix_graph_grouped is set to true
so that the following GC invocations can skip Tarjan's algorithm
and simply iterate through the list in unix_walk_scc_fast().

In addition, if we know there is at least one cyclic reference,
we set unix_graph_maybe_cyclic to true so that we do not skip GC.

So the state transitions as follows:

  (unix_graph_maybe_cyclic, unix_graph_grouped)
  =
  (false, false) -> (true, false) -> (true, true) or (false, true)
                         ^.______________/________________/

There is no transition to the initial state where both variables
are false.

If we consider the initial state as grouped, we can see that the
GC actually has a tristate.

Let's consolidate two variables into one enum.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251115020935.2643121-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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