oom_reaper: avoid pointless atomic_inc_not_zero usage.

Since commit 36324a990cf5 ("oom: clear TIF_MEMDIE after oom_reaper
managed to unmap the address space") changed to use find_lock_task_mm()
for finding a mm_struct to reap, it is guaranteed that mm->mm_users > 0
because find_lock_task_mm() returns a task_struct with ->mm != NULL.
Therefore, we can safely use atomic_inc().

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

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