mm/share_pool: Support read-only memory allocation

ascend inclusion
category: Bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I578LV


CVE: NA

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When the driver uses the shared pool memory to share the memory
with the user space, the user space is not allowed to operate
this area. This prevents users from damaging sensitive data.

When the sp_alloc and k2u processes apply for private memory,
read-only memory can be applied for.

Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>