mm/sharepool: Add base framework for share_pool

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



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The share pool features is a big feature, it is mainly used to share
user virtual memory for different processes in the same group.
It could be used by this steps:
1. Process A create a new group which is owned by process A.
2. Process A add process B to the group.
3. Process A add process C to the same group.
4. Process B alloc a new memory VA, and write something in it.
5. The VA was send to the process C by IPC, then process C got it.
6. The process C access the VA and got the data directly.
7. The process A could add more processes in the group to share the
memory.
8. Fix the memory by use the free function or exit the group.

The new features is enabled both by CONFIG_SHARE_POOL and the
enable_ascend_share_pool bootarg, it would not affect anything if disabled.

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