locktorture: Fix memory leak in param_set_cpumask()

With CONFIG_CPUMASK_OFFSTACK=y, the 'bind_writers' buffer is allocated via
alloc_cpumask_var() in param_set_cpumask(). But it is not freed, when
setting the module parameter multiple times by sysfs interface or removing
module.

Below kmemleak trace is seen for this issue:

unreferenced object 0xffff888100aabff8 (size 8):
  comm "bash", pid 323, jiffies 4295059233
  hex dump (first 8 bytes):
    07 00 00 00 00 00 00 00                          ........
  backtrace (crc ac50919):
    __kmalloc_node_noprof+0x2e5/0x420
    alloc_cpumask_var_node+0x1f/0x30
    param_set_cpumask+0x26/0xb0 [locktorture]
    param_attr_store+0x93/0x100
    module_attr_store+0x1b/0x30
    kernfs_fop_write_iter+0x114/0x1b0
    vfs_write+0x300/0x410
    ksys_write+0x60/0xd0
    do_syscall_64+0xa4/0x260
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

This issue can be reproduced by:
  insmod locktorture.ko bind_writers=1
  rmmod locktorture

or:
  insmod locktorture.ko bind_writers=1
  echo 2 > /sys/module/locktorture/parameters/bind_writers

Considering that setting the module parameter 'bind_writers' or
'bind_readers' by sysfs interface has no real effect, set the parameter
permissions to 0444. To fix the memory leak when removing module, free
'bind_writers' and 'bind_readers' memory in lock_torture_cleanup().

Fixes: 73e3412 ("locktorture: Add readers_bind and writers_bind module parameters")
Suggested-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Wang Liang <wangliang74@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: Wang Liang

kernel/locking/locktorture.c

@@ -103,8 +103,8 @@ static const struct kernel_param_ops lt_bind_ops = {
 	.get = param_get_cpumask,
 };
 
-module_param_cb(bind_readers, &lt_bind_ops, &bind_readers, 0644);
-module_param_cb(bind_writers, &lt_bind_ops, &bind_writers, 0644);
+module_param_cb(bind_readers, &lt_bind_ops, &bind_readers, 0444);
+module_param_cb(bind_writers, &lt_bind_ops, &bind_writers, 0444);
 
 long torture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask, bool dowarn);
 
@@ -1211,6 +1211,10 @@ static void lock_torture_cleanup(void)
 			cxt.cur_ops->exit();
 		cxt.init_called = false;
 	}
+
+	free_cpumask_var(bind_readers);
+	free_cpumask_var(bind_writers);
+
 	torture_cleanup_end();
 }