selftests: netfilter: add MTU arguments to flowtables

Add some documentation, default values defined in original
script and Originator/Link/Responder arguments
using getopts like in tools/power/cpupower/bench/cpufreq-bench_plot.sh

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>