mm: copy over oom_adj value at fork time

Fix a post-2.6.31 regression which was introduced by
2ff05b2 ("oom: move oom_adj value from
task_struct to mm_struct").

After moving the oom_adj value from the task struct to the mm_struct, the
oom_adj value was no longer properly inherited by child processes.

Copying over the oom_adj value at fork time fixes that bug.

[kosaki.motohiro@jp.fujitsu.com: test for current->mm before dereferencing it]
Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Paul Menage <manage@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -426,6 +426,7 @@ static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
 	init_rwsem(&mm->mmap_sem);
 	INIT_LIST_HEAD(&mm->mmlist);
 	mm->flags = (current->mm) ? current->mm->flags : default_dump_filter;
+	mm->oom_adj = (current->mm) ? current->mm->oom_adj : 0;
 	mm->core_state = NULL;
 	mm->nr_ptes = 0;
 	set_mm_counter(mm, file_rss, 0);