vsock/test: use send_buf() in vsock_test.c

We have a very common pattern used in vsock_test that we can
now replace with the new send_buf().

This allows us to reuse the code we already had to check the
actual return value and wait for all the bytes to be sent with
an appropriate timeout.

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