oom: fix possible oom_dump_tasks NULL pointer

When /proc/sys/vm/oom_dump_tasks is enabled, it is possible to get a NULL
pointer for tasks that have detached mm's since task_lock() is not held
during the tasklist scan.  Add the task_lock().

Acked-by: Nick Piggin <npiggin@suse.de>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
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>