oom: do not dump task state for non thread group leaders

When /proc/sys/vm/oom_dump_tasks is enabled, it's only necessary to dump
task state information for thread group leaders.  The kernel log gets
quickly overwhelmed on machines with a massive number of threads by
dumping non-thread group leaders.
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -295,6 +295,8 @@ static void dump_tasks(const struct mem_cgroup *mem)
 			continue;
 		if (mem && !task_in_mem_cgroup(p, mem))
 			continue;
+		if (!thread_group_leader(p))
+			continue;
 
 		task_lock(p);
 		printk(KERN_INFO "[%5d] %5d %5d %8lu %8lu %3d     %3d %s\n",