documentation of some IP/ICMP snmp counters

The snmp_counter.rst explains the meanings of snmp counters. It also
provides a set of experiments (only 1 for this initial patch),
combines the experiments' resutls and the snmp counters'
meanings. This is an initial path, only explains a part of IP/ICMP
counters and provide a simple ping test.

Signed-off-by: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>