vsock/test: Add test for UAF due to socket unbinding

Fail the autobind, then trigger a transport reassign. Socket might get
unbound from unbound_sockets, which then leads to a reference count
underflow.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250128-vsock-transport-vs-autobind-v3-5-1cf57065b770@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>