samples: pktgen: fix to print when terminated normally

Currently, most pktgen samples print the execution result when the
program is terminated normally. However, sample03 doesn't work
appropriately.

This is results of samples:

    # DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample04_many_flows.sh -n 1
    Running... ctrl^C to stop
    Device: eth0@0
    Result: OK: 19(c5+d13) usec, 1 (60byte,0frags)
    51762pps 24Mb/sec (24845760bps) errors: 0

    # DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample03_burst_single_flow.sh -n 1
    Running... ctrl^C to stop

The reason why it doesn't print the execution result when the program is
terminated usually is that sample03 doesn't call the function which
prints the result, unlike other samples.

So, this commit solves this issue by calling the function before
termination. Also, this commit changes control_c function to
print_result to maintain consistency with other samples.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>