af_unix: Avoid Tarjan's algorithm if unnecessary.

Once a cyclic reference is formed, we need to run GC to check if
there is dead SCC.

However, we do not need to run Tarjan's algorithm if we know that
the shape of the inflight graph has not been changed.

If an edge is added/updated/deleted and the edge's successor is
inflight, we set false to unix_graph_grouped, which means we need
to re-classify SCC.

Once we finalise SCC, we set true to unix_graph_grouped.

While unix_graph_grouped is true, we can iterate the grouped
SCC using vertex->scc_entry in unix_walk_scc_fast().

list_add() and list_for_each_entry_reverse() uses seem weird, but
they are to keep the vertex order consistent and make writing test
easier.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-12-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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