mm/sharepool: Protect the va reserved for sharepool

ascend inclusion
category: Feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LNGH



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Add protections for the va reserved for sharepool. Forbid mremap/munmap
to access that range.

Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>