acct: serialize acct_on()

brute-force - on a global mutex that isn't nested into anything.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/acct.c

@@ -241,6 +241,8 @@ static int acct_on(struct filename *pathname)
 	return 0;
 }
 
+static DEFINE_MUTEX(acct_on_mutex);
+
 /**
  * sys_acct - enable/disable process accounting
  * @name: file name for accounting records or NULL to shutdown accounting
@@ -263,7 +265,9 @@ SYSCALL_DEFINE1(acct, const char __user *, name)
 		struct filename *tmp = getname(name);
 		if (IS_ERR(tmp))
 			return PTR_ERR(tmp);
+		mutex_lock(&acct_on_mutex);
 		error = acct_on(tmp);
+		mutex_unlock(&acct_on_mutex);
 		putname(tmp);
 	} else {
 		struct bsd_acct_struct *acct;