mm, oom: show process exiting information in __oom_kill_process()

mm, oom: show process exiting information in __oom_kill_process()

When the OOM killer finds a victim and tryies to kill it, if the victim is
already exiting, the task mm will be NULL and no process will be killed.
But the dump_header() has been already executed, so it will be strange to
dump so much information without killing a process.  We'd better show some
helpful information to indicate why this happens.

Suggested-by: David Rientjes <[email protected]>
Signed-off-by: Yafang Shao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Qian Cai <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>