We already get the attibutes of ifprop during getifprop() afterreading /proc/net/dev, thus remove the unnecessary second getterin deviatsyst().Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

The network devices, like eth2, are settled in another networknamespace via `ip netns add` in some scenarios in the productionenvironment. In this case, we have to switch to that new netnsto get network related statistics, e.g. reading from /proc/net/dev,/proc/net/snmp, /proc/net/snmp6.This patch firstly adds the needed attributes, like 'netns', forcollecting and displaying network namespace statistics. Then thefollowing patches will do the real implementation.To pass the compiling, also update for atopsar, parsable and json.Also format some lines according to astyle.Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

For linux kernel, it exposes almost 100 lines for /proc/net/snmp6file. Considering multiple-netns situations, too many lines had tobe read and transferred, which is quite time consuming and spaceconsuming.Let's optimize this by adding 'idx' to mark the needed indicators'position in struct v6tab. During the first time reading, e.i. theinit_net namespace, we record the positions. Then in the followingother netns, we only need to store and transfer indicators' valuesaccording to their positions.Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

This patch implements how to collect statistics for other netnsby using `setns` to switch to another netns to read /proc files,and then use shared memory for parent-child IPC.Combined with actual needs and performance considerations, thispatch only collects other netns's physical network devices stats.Virtual devices are ignored except the 'lo'.Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

The default rule is splitting netns by blank space.And make sure the netns path exits.Signed-off-by: Fei Li <lifei.shirley@bytedance.com>