af_unix: Set drop reason in __unix_gc().

Inflight file descriptors by SCM_RIGHTS hold references to the
struct file.

AF_UNIX sockets could hold references to each other, forming
reference cycles.

Once such sockets are close()d without the fd recv()ed, they
will be unaccessible from userspace but remain in kernel.

__unix_gc() garbage-collects skb with the dead file descriptors
and frees them by __skb_queue_purge().

Let's set SKB_DROP_REASON_SOCKET_CLOSE there.

  # echo 1 > /sys/kernel/tracing/events/skb/kfree_skb/enable

  # python3
  >>> from socket import *
  >>> from array import array
  >>>
  >>> # Create a reference cycle
  >>> s1 = socket(AF_UNIX, SOCK_DGRAM)
  >>> s1.bind('')
  >>> s1.sendmsg([b"nop"], [(SOL_SOCKET, SCM_RIGHTS, array("i", [s1.fileno()]))], 0, s1.getsockname())
  >>> s1.close()
  >>>
  >>> # Trigger GC
  >>> s2 = socket(AF_UNIX)
  >>> s2.close()

  # cat /sys/kernel/tracing/trace_pipe
  ...
     kworker/u16:2-42 ... kfree_skb: ... location=__unix_gc+0x4ad/0x580 reason: SOCKET_CLOSE

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250116053441.5758-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/garbage.c

@@ -573,7 +573,7 @@ static void __unix_gc(struct work_struct *work)
 			UNIXCB(skb).fp->dead = true;
 	}
 
-	__skb_queue_purge(&hitlist);
+	__skb_queue_purge_reason(&hitlist, SKB_DROP_REASON_SOCKET_CLOSE);
 skip_gc:
 	WRITE_ONCE(gc_in_progress, false);
 }