High: IPaddr2/findif.sh: Robust parameter checking for 'nic'

A false failure detection may happen if the following conditions are met:

• the kernel is 2.6.18 or prior
• two or more processes can read /proc/net/dev at the same time
• reading the proc file using 'read' of bash (or lseek()) line by line

See this issue for the details:
https://github.com/ClusterLabs/resource-agents/pull/320