mm,oom_reaper: don't call mmput_async() without atomic_inc_not_zero()

Commit e2fe14564d33 ("oom_reaper: close race with exiting task") reduced
frequency of needlessly selecting next OOM victim, but was calling
mmput_async() when atomic_inc_not_zero() failed.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tetsuo Handa <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/oom_kill.c

···
 	mm = p->mm;
 	if (!atomic_inc_not_zero(&mm->mm_users)) {
 		task_unlock(p);
+		mm = NULL;
 		goto unlock_oom;
 	}