mm/sharepool: use rwsem to protect sp group exit

ascend inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5DS9S


CVE: NA

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Fix following situation:

when the last process in a group exits, and a second process tries to add
to this group.

The second process may get a invalid spg. However the group's
use_count is increased by 1, which caused the first process failed to
free the group when it exits. And then second process called
sp_group_drop --> free_sp_group and cause a double request of rwsem.

Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>