[Bug] Failed to get TCP metrics when linux kernel is 2.x

[morningman]

Describe the bug
In PR #3835, I get the tcp metrics from /proc/net/snmp.
But the content of /proc/net/snmp is different in linux kernel 2.x and kernel 3.x

In 2.x, the content of tcp is like:

Tcp: RtoAlgorithm RtoMin RtoMax MaxConn ActiveOpens PassiveOpens AttemptFails EstabResets CurrEstab InSegs OutSegs RetransSegs InErrs OutRsts
Tcp: 1 200 120000 -1 31802292 26291079 5229284 1561770 327 3168520693 3130748602 4190670 0 19075525

In 3.x, the content of tcp is like:

Tcp: RtoAlgorithm RtoMin RtoMax MaxConn ActiveOpens PassiveOpens AttemptFails EstabResets CurrEstab InSegs OutSegs RetransSegs InErrs OutRsts InCsumErrors
Tcp: 1 200 120000 -1 58962964 46828036 4108757 3626498 204 1242409051 1436033565 1146772 17207 30277468 0

You can see that in 3.x, there is addition metric InCsumErrors at the end.

So we should be able to get the metrics from different content.