zdtm: use pr_err if errno isn't required

205             if (fd < 0) {
>>> >>>     CID 175317:    (PW.BAD_PRINTF_FORMAT_STRING)
>>> >>>     invalid format string conversion
206                     pr_perror("can't accept client connection %m");

travis-ci: success for zdtm: use pr_err if errno isn't required
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>