vhost/vsock: always initialize seqpacket_allow

In the Linux kernel, the following vulnerability has been resolved:

vhost/vsock: always initialize seqpacket_allow

There are two issues around seqpacket_allow:

1. seqpacket_allow is not initialized when socket is

created. Thus if features are never set, it will be

read uninitialized.

2. if VIRTIO_VSOCK_F_SEQPACKET is set and then cleared,

then seqpacket_allow will not be cleared appropriately

(existing apps I know about don't usually do this but

it's legal and there's no way to be sure no one relies

on this).

To fix:

- initialize seqpacket_allow after allocation

- set it unconditionally in set_features