vhost: fix potential fd leak

vhost: fix potential fd leak

Vhost will create temporary file when receiving VHOST_USER_GET_INFLIGHT_FD
message. Malicious guest can send endless this message to drain out the
resource of host.

When receiving VHOST_USER_GET_INFLIGHT_FD message repeatedly, closing the
file created during the last handling of this message.

CVE-2020-10726
Fixes: d87f1a1cb7b666550 ("vhost: support inflight info sharing")
Cc: stable@dpdk.org
Signed-off-by: Xuan Ding <xuan.ding@intel.com>
Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>