test/perf: Explicitly control UDP packet length, instead of MTU

Packet size can make a big difference to UDP throughput, so it makes sense
to measure it for a variety of different sizes.  Currently we do this by
adjusting the MTU on the relevant interface before running iperf3.

However, the UDP packet size has no inherent connection to the MTU - it's
controlled by the sender, and the MTU just affects whether the packet will
make it through or be fragmented.  The only reason adjusting the MTU works
is because iperf3 bases its default packet size on the (path) MTU.

We can test this more simply by using the -l option to the iperf3 client
to directly control the packet size, instead of adjusting the MTU.

As well as simplifying this lets us test different packet sizes for host to
ns traffic.  We couldn't do that previously because we don't have
permission to change the MTU on the host.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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