selftests/net: udpgso: test small gso_size boundary conditions

Verify that udpgso can generate segments smaller than device mtu, down
to the extreme case of 1B gso_size.

Verify that irrespective of gso_size, udpgso restricts the number of
segments it will generate per call (UDP_MAX_SEGMENTS).

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>