vsock: Fix sk_error_queue memory leak

Kernel queues MSG_ZEROCOPY completion notifications on the error queue.
Where they remain, until explicitly recv()ed. To prevent memory leaks,
clean up the queue when the socket is destroyed.

unreferenced object 0xffff8881028beb00 (size 224):
  comm "vsock_test", pid 1218, jiffies 4294694897
  hex dump (first 32 bytes):
    90 b0 21 17 81 88 ff ff 90 b0 21 17 81 88 ff ff  ..!.......!.....
    00 00 00 00 00 00 00 00 00 b0 21 17 81 88 ff ff  ..........!.....
  backtrace (crc 6c7031ca):
    [<ffffffff81418ef7>] kmem_cache_alloc_node_noprof+0x2f7/0x370
    [<ffffffff81d35882>] __alloc_skb+0x132/0x180
    [<ffffffff81d2d32b>] sock_omalloc+0x4b/0x80
    [<ffffffff81d3a8ae>] msg_zerocopy_realloc+0x9e/0x240
    [<ffffffff81fe5cb2>] virtio_transport_send_pkt_info+0x412/0x4c0
    [<ffffffff81fe6183>] virtio_transport_stream_enqueue+0x43/0x50
    [<ffffffff81fe0813>] vsock_connectible_sendmsg+0x373/0x450
    [<ffffffff81d233d5>] ____sys_sendmsg+0x365/0x3a0
    [<ffffffff81d246f4>] ___sys_sendmsg+0x84/0xd0
    [<ffffffff81d26f47>] __sys_sendmsg+0x47/0x80
    [<ffffffff820d3df3>] do_syscall_64+0x93/0x180
    [<ffffffff8220012b>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 581512a6 ("vsock/virtio: MSG_ZEROCOPY flag support")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>