[LTP] [PATCH 1/1] if-mtu-change.sh: Add max packet size detection for IPv4

[Petr Vorel]

Hi Alexey,

> > > -		tst_ping -s "1 1000 65507"
> > > +		tst_ping -s "1 1000 $MAX_PACKET_SIZE"

> > tst_ping -s "1 $((mtu / 2)) $mtu $MAX_PACKET_SIZE"?
> Thinking about this twice: adding forth size prolong test a bit (test is already
> quite slow. But it makes sense to me to test all 4 variants.

Merged the original version. I wonder if we can change the default
CHANGE_INTERVAL to 3 or even less (1) to make testing a bit faster.
Is the value more important for two host based testing?
Because it works on netns based testing.

Kind regards,
Petr