af_unix: Count cyclic SCC.

af_unix: Count cyclic SCC.

__unix_walk_scc() and unix_walk_scc_fast() call unix_scc_cyclic()
for each SCC to check if it forms a cyclic reference, so that we
can skip GC at the following invocations in case all SCCs do not
have any cycles.

If we count the number of cyclic SCCs in __unix_walk_scc(), we can
simplify unix_walk_scc_fast() because the number of cyclic SCCs
only changes when it garbage-collects a SCC.

So, let's count cyclic SCC in __unix_walk_scc() and decrement it
in unix_walk_scc_fast() when performing garbage collection.

Note that we will use this counter in a later patch to check if a
cycle existed in the previous GC run.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251115020935.2643121-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>