[benchmarks] fix using of sockets for gawk 3.1.8

http://lists.gnu.org/archive/html/info-gnu/2011-06/msg00013.html

15. Support for IPv6 is added via the /inet6/... special file.
/inet4/... forces IPv4 and /inet chooses the system default (probably
IPv4).

It means that /inet4/ available from gawk 4.0.0, in older version there
is /inet/ only