acct: eliminate compile warning

If ACCT_VERSION is not defined to 3, below warning appears:
  CC      kernel/acct.o
  kernel/acct.c: In function `do_acct_process':
  kernel/acct.c:475:24: warning: unused variable `ns' [-Wunused-variable]

[akpm@linux-foundation.org: retain the local for code size improvements
Signed-off-by: NYing Xue <ying.xue@windriver.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/acct.c

@@ -472,7 +472,6 @@ static void do_acct_process(struct bsd_acct_struct *acct)
 	acct_t ac;
 	unsigned long flim;
 	const struct cred *orig_cred;
-	struct pid_namespace *ns = acct->ns;
 	struct file *file = acct->file;
 
 	/*
@@ -500,10 +499,15 @@ static void do_acct_process(struct bsd_acct_struct *acct)
 	ac.ac_gid16 = ac.ac_gid;
 #endif
 #if ACCT_VERSION == 3
-	ac.ac_pid = task_tgid_nr_ns(current, ns);
-	rcu_read_lock();
-	ac.ac_ppid = task_tgid_nr_ns(rcu_dereference(current->real_parent), ns);
-	rcu_read_unlock();
+	{
+		struct pid_namespace *ns = acct->ns;
+
+		ac.ac_pid = task_tgid_nr_ns(current, ns);
+		rcu_read_lock();
+		ac.ac_ppid = task_tgid_nr_ns(rcu_dereference(current->real_parent),
+					     ns);
+		rcu_read_unlock();
+	}
 #endif
 	/*
 	 * Get freeze protection. If the fs is frozen, just skip the write