Improve Linux performance of GetAllInterfaces

[wjordan]

This Linux-specific optimization avoids a separate Netlink dump of addresses for each individual interface, which is prohibitively slow on servers with a large number (thousands) of network interfaces.

Here are some example test results on a sample server using the sockaddr eval command (which provides sockaddr.GetAllInterfaces() as template data), showing a ~100x performance improvement:

$ ls /sys/class/net | wc -l 3202 $ time ./sockaddr-orig eval 0 0 real	1m3.956s user	0m36.514s sys	0m55.716s $ time ./sockaddr-opt eval 0 0 real	0m0.457s user	0m0.096s sys	0m0.443s 

See golang/go#53660 for further discussion of this Linux performance issue with net.Interface.

[dangra]

+1 to Merge. It is a useful performance fix when dealing with tons of interfaces.