Fix update_net_stats() using sysctl() directly

At some point between dfly 3.4 and 3.5 getifaddrs() gave no stats
anymore; libc getifaddrs() (at least in dfly) uses PF_ROUTE mib part
while dfly /usr/src/tools/tools/ifinfo/ifinfo.c PF_LINK one.

Just make like ifinfo.c, should be the right one