vsock/test: fix seqpacket message bounds test

vsock/test: fix seqpacket message bounds test

The test requires the sender (client) to send all messages before waking
up the receiver (server).
Since virtio-vsock had a bug and did not respect the size of the TX
buffer, this test worked, but now that we are going to fix the bug, the
test hangs because the sender would fill the TX buffer before waking up
the receiver.

Set the buffer size in the sender (client) as well, as we already do for
the receiver (server).

Fixes: 5c338112e48a ("test/vsock: rework message bounds test")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260121093628.9941-3-sgarzare@redhat.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>