samples: pktgen: fix append mode failed issue

Each sample script sources functions.sh before parameters.sh
which makes $APPEND undefined when trapping EXIT no matter in
append mode or not. Due to this when sample scripts finished
they always do "pgctrl reset" which resets pktgen config.

So move trap to each script after sourcing parameters.sh
and trap EXIT explicitly.

Signed-off-by: J.J. Martzki <mars14850@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>