virtio/vsock: Fix accept_queue memory leak

virtio/vsock: Fix accept_queue memory leak

As the final stages of socket destruction may be delayed, it is possible
that virtio_transport_recv_listen() will be called after the accept_queue
has been flushed, but before the SOCK_DONE flag has been set. As a result,
sockets enqueued after the flush would remain unremoved, leading to a
memory leak.

vsock_release
  __vsock_release
    lock
    virtio_transport_release
      virtio_transport_close
        schedule_delayed_work(close_work)
    sk_shutdown = SHUTDOWN_MASK
(!) flush accept_queue
    release
                                        virtio_transport_recv_pkt
                                          vsock_find_bound_socket
                                          lock
                                          if flag(SOCK_DONE) return
                                          virtio_transport_recv_listen
                                            child = vsock_create_connected
                                      (!)   vsock_enqueue_accept(child)
                                          release
close_work
  lock
  virtio_transport_do_close
    set_flag(SOCK_DONE)
    virtio_transport_remove_sock
      vsock_remove_sock
        vsock_remove_bound
  release

Introduce a sk_shutdown check to disallow vsock_enqueue_accept() during
socket destruction.

unreferenced object 0xffff888109e3f800 (size 2040):
  comm "kworker/5:2", pid 371, jiffies 4294940105
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    28 00 0b 40 00 00 00 00 00 00 00 00 00 00 00 00  (..@............
  backtrace (crc 9e5f4e84):
    [<ffffffff81418ff1>] kmem_cache_alloc_noprof+0x2c1/0x360
    [<ffffffff81d27aa0>] sk_prot_alloc+0x30/0x120
    [<ffffffff81d2b54c>] sk_alloc+0x2c/0x4b0
    [<ffffffff81fe049a>] __vsock_create.constprop.0+0x2a/0x310
    [<ffffffff81fe6d6c>] virtio_transport_recv_pkt+0x4dc/0x9a0
    [<ffffffff81fe745d>] vsock_loopback_work+0xfd/0x140
    [<ffffffff810fc6ac>] process_one_work+0x20c/0x570
    [<ffffffff810fce3f>] worker_thread+0x1bf/0x3a0
    [<ffffffff811070dd>] kthread+0xdd/0x110
    [<ffffffff81044fdd>] ret_from_fork+0x2d/0x50
    [<ffffffff8100785a>] ret_from_fork_asm+0x1a/0x30

Fixes: 3fe356d58efa ("vsock/virtio: discard packets only when socket is really closed")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>