We need to reset metadata cache during new IOTLB initialization,
otherwise the stale pointers to previous IOTLB may be still accessed
which will lead a use after free.

Reported-by: syzbot+c51e6736a1bf614b3272@syzkaller.appspotmail.com
Fixes: f8894913 ("vhost: introduce O(1) vq metadata cache")
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>