selftest/bpf: Adapt vsock_delete_on_close to sockmap rejecting unconnected

Commit 51574544 ("selftest/bpf: Add test for vsock removal from sockmap
on close()") added test that checked if proto::close() callback was invoked
on AF_VSOCK socket release. I.e. it verified that a close()d vsock does
indeed get removed from the sockmap.

It was done simply by creating a socket pair and attempting to replace a
close()d one with its peer. Since, due to a recent change, sockmap does not
allow updating index with a non-established connectible vsock, redo it with
a freshly established one.

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