selftests: netfilter: exit on invalid parameters

exit script with comments when parameters are wrong during address
addition. No need for a message when trying to change MTU with lower
values: output is self-explanatory.
Use short testing sequence to avoid shellcheck warnings
(suggested by Stefano Brivio).

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