vsock/test: add a final full barrier after run all tests

vsock/test: add a final full barrier after run all tests

If the last test fails, the other side still completes correctly,
which could lead to false positives.

Let's add a final barrier that ensures that the last test has finished
correctly on both sides, but also that the two sides agree on the
number of tests to be performed.

Fixes: 2f65b44e199c ("VSOCK: add full barrier between test cases")
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260108114419.52747-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>