[v4.9, v4.14] vhost/vsock: fix uninitialized vhost_vsock->guest_cid

Please apply mainline commit a72b69dc083a931422cc8a5e33841aff7d5312f2 ("vhost/vsock: fix uninitialized vhost_vsock->guest_cid") to the v4.9 and v4.14 stable branches.

I believe this is the root cause of an issue uncovered by applying "vhost/vsock: fix use-after-free in network stack callers" in these branches. I sometimes see a crash in hash_del_rcu() with vsock in the call stack, and that call is protected by a newly-added check of vsock->guest_cid, which was uninitialized before this commit.

v4.4 doesn't have vsock, and v4.19 already has this commit, so they don't need to be fixed.

Thanks, -- Daniel