mm: make set_mempolicy(MPOL_INTERLEAV) N_HIGH_MEMORY aware

At first, init_task's mems_allowed is initialized as this.
 init_task->mems_allowed == node_state[N_POSSIBLE]

And cpuset's top_cpuset mask is initialized as this
 top_cpuset->mems_allowed = node_state[N_HIGH_MEMORY]

Before 2.6.29:
policy's mems_allowed is initialized as this.

  1. update tasks->mems_allowed by its cpuset->mems_allowed.
  2. policy->mems_allowed = nodes_and(tasks->mems_allowed, user's mask)

Updating task's mems_allowed in reference to top_cpuset's one.
cpuset's mems_allowed is aware of N_HIGH_MEMORY, always.

In 2.6.30: After commit 58568d2a
("cpuset,mm: update tasks' mems_allowed in time"), policy's mems_allowed
is initialized as this.

  1. policy->mems_allowd = nodes_and(task->mems_allowed, user's mask)

Here, if task is in top_cpuset, task->mems_allowed is not updated from
init's one.  Assume user excutes command as #numactrl --interleave=all
,....

  policy->mems_allowd = nodes_and(N_POSSIBL...