[linux] fix /proc/net/dev parsing

Fix parsing for centos 6.4: 
```
cat /proc/net/dev 
Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
    lo:30593187   23551    0    0    0     0          0         0 30593187   23551    0    0    0     0       0          0
  eth1:79088109  201292    0    0    0     0          0         0 236038842  234067    0    0    0     0       0          0
```