vsock/test: add recv_buf() utility function

Move the code of recv_byte() out in a new utility function that
can be used to receive a generic buffer.

This new function can be used when we need to receive a custom
buffer and not just a single 'A' byte.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Signed-off-by: David S. Miller <davem@davemloft.net>