CVE-2024-43873 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