show_schedstat(): fix memleak

The Coverity checker spotted a memleak introduced by commit
39106dcf (cpumask: use new cpus_scnprintf
function).

It seems the kfree() got lost between v2 and v3 of this patch...
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: Mike Travis <travis@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_stats.h

@@ -67,6 +67,7 @@ static int show_schedstat(struct seq_file *seq, void *v)
 		preempt_enable();
 #endif
 	}
+	kfree(mask_str);
 	return 0;
 }