benchtest: Correct shell script related to bench-malloc-thread

benchtest: Correct shell script related to bench-malloc-thread

This patch changes the shell script that selects which arguments are used
for the execution of bench-malloc-thread.
The problem seems to have been introduced in commit:

  commit 2d6427a63cad8056ba6bcaaaa8df21977c8dde3d
  Author: Wangyang Guo <wangyang.guo@intel.com>
  Date:   Fri Nov 29 16:05:35 2024 +0800
  benchtests: Add calloc test

With current condition, the following error "/bin/sh: 3: [[: not found"
occurs when executing `make bench BENCHSET="malloc-thread"` and the else
path is taken, using incorrect arguments for bench test execution.

Error is reproducible in Debian based distros.

Reviewed-by: Florian Weimer <fweimer@redhat.com>