tcpdump01: fix the way to identify network interface

The script tries to identify network interface that can communicate with RHOST
by searching for the IP of RHOST in ifconfig output on localhost, which doesn't
make any sense when RHOST is not localhost.  This patch changes the script to
indetify the network interface by ping RHOST.

Signed-off-by: Simon Xu <xu.simon@oracle.com>