samples: pktgen: pass the environment variable of normal user to sudo

samples: pktgen: pass the environment variable of normal user to sudo

All pktgen samples can use the environment variable instead of option
parameters(eg. $DEV is able to use instead of '-i' option).

This is results of running sample as root and user:

    // running as root
    # DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample01_simple.sh -v -n 1
    Running... ctrl^C to stop

    // running as normal user
    $ DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample01_simple.sh -v -n 1
    [...]
    ERROR: Please specify output device

This results show the sample doesn't work properly when the sample runs
as normal user. Because the sample is restarted by the function
(root_check_run_with_sudo) to run with sudo. In this process, the
environment variable of normal user doesn't propagate to sudo.

It can be solved by using "-E"(--preserve-env) option of "sudo", which
preserve normal user's existing environment variables. So this commit
adds "-E" option in the function (root_check_run_with_sudo).

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