af_unix: fix listen() succeeding on sockets in the wrong state

af_unix: fix listen() succeeding on sockets in the wrong state

Commit fd0a109a0f6b ("net, pidfs: prepare for handing out pidfds for
reaped sk->sk_peer_pid") inserted a prepare_peercred() call between err
= -EINVAL and the socket-state check in unix_listen(). Since
prepare_peercred() leaves err at 0 on success, listen() on an AF_UNIX
socket that is not in TCP_CLOSE or TCP_LISTEN state (e.g. one that is
already connected) now silently returns success without doing anything,
instead of failing with EINVAL as it did before.

Fixes: fd0a109a0f6b ("net, pidfs: prepare for handing out pidfds for reaped sk->sk_peer_pid")
Signed-off-by: John Ericson <mail@johnericson.me>
Link: https://patch.msgid.link/20260718182903.2295560-1-John.Ericson@Obsidian.Systems
Signed-off-by: Jakub Kicinski <kuba@kernel.org>