af_unix: Don't trigger GC from close() if unnecessary.

af_unix: Don't trigger GC from close() if unnecessary.

We have been triggering GC on every close() if there is even one
inflight AF_UNIX socket.

This is because the old GC implementation had no idea of the graph
shape formed by SCM_RIGHTS references.

The new GC knows whether there could be a cyclic reference or not,
and we can do better.

Let's not trigger GC from close() if there is no cyclic reference
or GC is already in progress.

While at it, unix_gc() is renamed to unix_schedule_gc() as it does
not actually perform GC since commit 8b90a9f819dc ("af_unix: Run
GC on only one CPU.").

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251115020935.2643121-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>