selftest/bpf: Add vsock test for sockmap rejecting unconnected

Verify that for a connectible AF_VSOCK socket, merely having a transport
assigned is insufficient; socket must be connected for the sockmap to
accept.

This does not test datagram vsocks. Even though it hardly matters. VMCI is
the only transport that features VSOCK_TRANSPORT_F_DGRAM, but it has an
unimplemented vsock_transport::readskb() callback, making it unsupported by
BPF/sockmap.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>