mm: Exclude CDM nodes from task->mems_allowed and root cpuset

ascend inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4JMLR


CVE: NA
-------------------

Task struct's mems_allowed element decides the final nodemask from which
memory can be allocated in the task context irrespective any applicable
memory policy. CDM nodes should not be used for user allocations, its one
of the overall requirements of it's isolation. So they should not be part
of any task's mems_allowed nodemask. System RAM nodemask is used instead
of node_states[N_MEMORY] nodemask during mems_allowed initialization and
it's update during memory hotlugs.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Lijun Fang <fanglijun3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>