af_unix: Save listener for embryo socket.

This is a prep patch for the following change, where we need to
fetch the listening socket from the successor embryo socket
during GC.

We add a new field to struct unix_sock to save a pointer to a
listening socket.

We set it when connect() creates a new socket, and clear it when
accept() is called.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-8-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/af_unix.h

@@ -83,6 +83,7 @@ struct unix_sock {
 	struct path		path;
 	struct mutex		iolock, bindlock;
 	struct sock		*peer;
+	struct sock		*listener;
 	struct unix_vertex	*vertex;
 	struct list_head	link;
 	unsigned long		inflight;