Bluetooth: RFCOMM: avoid socket lock inversion in listener cleanup

rfcomm_sock_cleanup_listen() closes unaccepted child sockets through
rfcomm_sock_close(), which takes the child socket lock before
rfcomm_dlc_close() acquires rfcomm_mutex. The RFCOMM worker takes these
locks in reverse order while handling connections and DLC state changes,
so lockdep reports a possible deadlock.

Close dequeued children without taking their socket lock. The accept queue
owns a reference to each child, and bt_accept_dequeue() locks the child
while unlinking it and clearing its parent pointer.

Dropping the child lock makes it important to prevent a concurrent
rfcomm_connect_ind() from enqueueing a new child after cleanup observes an
empty queue. Set a listening socket to BT_CLOSED while its lock is still
held, before dropping the lock and draining the queue. The state check in
rfcomm_connect_ind() then rejects new children once cleanup starts.

Reported-by: syzbot+0cece8fa7d83523f47a3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0cece8fa7d83523f47a3
Fixes: b7ce436a5d79 ("Bluetooth: switch to lock_sock in RFCOMM")
Signed-off-by: Juan Perdomo <jcperdomo100@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>