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>